Fix build failure with GCC 11:

------
[...]
/tmp/guix-build-opencolorio-1.1.1.drv-0/source/src/core/ImageDesc.cpp:60:51: error: ‘this’ pointer is null [-Werror=nonnull]
   60 |             os << "width=" << packedImg->getWidth() << ", ";
      |                                                   ^
/tmp/guix-build-opencolorio-1.1.1.drv-0/source/src/core/ImageDesc.cpp:274:10: note: in a call to non-static member function ‘long int OpenColorIO::v1::PackedImageDesc::getWidth() const’
  274 |     long PackedImageDesc::getWidth() const
      |          ^~~~~~~~~~~~~~~
/tmp/guix-build-opencolorio-1.1.1.drv-0/source/src/core/ImageDesc.cpp:61:53: error: ‘this’ pointer is null [-Werror=nonnull]
   61 |             os << "height=" << packedImg->getHeight() << ", ";
      |                                                     ^
/tmp/guix-build-opencolorio-1.1.1.drv-0/source/src/core/ImageDesc.cpp:279:10: note: in a call to non-static member function ‘long int OpenColorIO::v1::PackedImageDesc::getHeight() const’
  279 |     long PackedImageDesc::getHeight() const
      |          ^~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
[...]
------

Patch copied from Gentoo:

https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=7e726d850502018b6760da78dbd4a419603016b8

diff --git a/src/core/CMakeLists.txt b/src/core/CMakeLists.txt
index 1eb691b6..cff9bd83 100644
--- a/src/core/CMakeLists.txt
+++ b/src/core/CMakeLists.txt
@@ -23,8 +23,6 @@ if(WIN32)
     if("${CMAKE_BUILD_TYPE}" STREQUAL "Release")
         set(EXTERNAL_COMPILE_FLAGS "${EXTERNAL_COMPILE_FLAGS} /WX")
     endif()
-else()
-    set(EXTERNAL_COMPILE_FLAGS "${EXTERNAL_COMPILE_FLAGS} -Werror")
 endif()
 
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<div class='path'>path: <a href='/guix/log/?id=ec0a2444176bef974809c15ad4c6acb7f5423ac1'>root</a>/<a href='/guix/log/etc?id=ec0a2444176bef974809c15ad4c6acb7f5423ac1'>etc</a>/<a href='/guix/log/etc/snippets?id=ec0a2444176bef974809c15ad4c6acb7f5423ac1'>snippets</a>/<a href='/guix/log/etc/snippets/scheme-mode?id=ec0a2444176bef974809c15ad4c6acb7f5423ac1'>scheme-mode</a>/<a href='/guix/log/etc/snippets/scheme-mode/guix-origin?id=ec0a2444176bef974809c15ad4c6acb7f5423ac1'>guix-origin</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/etc/snippets/scheme-mode/guix-origin?id=ec0a2444176bef974809c15ad4c6acb7f5423ac1&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-06-26 22:45:28 +0200'>2021-06-26</span></td><td><a href='/guix/commit/etc/snippets/scheme-mode/guix-origin?id=d822c592438d025107c95dc49435a4c9c3cfd863'>etc: snippets: Use ‘hg-file-name’ when origin uses ‘hg-fetch’</a><span class='msg-avail'>...<span class='msg-tooltip'>Adjust to changes in commit aaafd19bd1e37265de07e246286a6819792c25b4.

* etc/snippets/scheme-mode/guix-origin: Use ‘hg-file-name’ instead of
‘string-append’ when ‘method’ for origin is ‘hg-fetch’.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Xinglu Chen</td></tr>
<tr><td><span title='2019-05-09 00:22:37 -0400'>2019-05-09</span></td><td><a href='/guix/commit/etc/snippets/scheme-mode/guix-origin?id=4ac69ea10f54384ac893313ef738e5ad554ac328'>Add (guix bzr-download).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/bzr-download.scm, guix/build/bzr.scm,
etc/snippets/scheme-mode/guix-bzr-reference: New files.
* Makefile.am (MODULES): Add them.
* etc/snippets/scheme-mode/guix-origin: Add "bzr-fetch" to the origin choices.
</span></span></td><td>Maxim Cournoyer</td></tr>