Fix a string comparison in C++ 11.

Taken from Debian:
https://sources.debian.org/src/dbus-c++/0.9.0-8.1/debian/patches/06_fix_gcc-7_ftbfs.patch/

--- a/src/pipe.cpp
+++ b/src/pipe.cpp
@@ -83,5 +83,5 @@ ssize_t Pipe::read(void *buffer, unsigne
 void Pipe::signal()
 {
   // TODO: ignoring return of read/write generates warning; maybe relevant for eventloop work...
-  ::write(_fd_write, '\0', 1);
+  ::write(_fd_write, "\0", 1);
 }
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<tr><td><span title='2020-01-15 18:40:43 +0100'>2020-01-15</span></td><td><a href='/guix/commit/tests/cpan.scm?id=4aea90b1876179aab8d603a42533a6bdf97ccd3c'>import: cpan: Rewrite tests to use an HTTP server instead of mocking.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-01-15 18:40:43 +0100'>2020-01-15</span></td><td><a href='/guix/commit/tests/cpan.scm?id=69f132554c6bd23df4610a21e636bde5f0578174'>import: cpan: Rewrite to use 'define-json-mapping'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-08-22 14:24:11 -0400'>2019-08-22</span></td><td><a href='/guix/commit/tests/cpan.scm?id=d020821c0bd2206a5f3d4db155f2a9a3de7dc670'>Revert "import: cpan: Adapt for the change to guile-json version 3."</a><span class='msg-avail'>...</span></td><td>Mark H Weaver</td></tr>
<tr><td><span title='2019-08-21 18:29:30 +0100'>2019-08-21</span></td><td><a href='/guix/commit/tests/cpan.scm?id=01ce7af25add55514f737af48ea6c127bedfde67'>import: cpan: Adapt for the change to guile-json version 3.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>