Add missing include that triggers a build failure on PowerPC:

  ../../../../gcc-5.4.0/libsanitizer/asan/asan_linux.cc: In function ‘bool __asan::AsanInterceptsSignal(int)’:
  ../../../../gcc-5.4.0/libsanitizer/asan/asan_linux.cc:222:20: error: ‘SIGSEGV’ was not declared in this scope
     return signum == SIGSEGV && common_flags()->handle_segv;
		      ^
From <https://patchwork.ozlabs.org/patch/725596/>.

diff --git a/libsanitizer/asan/asan_linux.cc b/libsanitizer/asan/asan_linux.cc
index c504168..59087b9 100644
--- a/libsanitizer/asan/asan_linux.cc
+++ b/libsanitizer/asan/asan_linux.cc
@@ -29,6 +29,7 @@
 #include <dlfcn.h>
 #include <fcntl.h>
 #include <pthread.h>
+#include <signal.h>
 #include <stdio.h>
 #include <unistd.h>
 #include <unwind.h>
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<tr><td><span title='2020-07-25 19:11:36 +0200'>2020-07-25</span></td><td><a href='/guix/commit/gnu/tests.scm?id=a5e2fc73760a2ae023f2e56bdbf8025971f90e64'>utils: Move &lt;location&gt; and '&amp;error-location' to (guix diagnostics).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-03-05 16:33:37 +0100'>2020-03-05</span></td><td><a href='/guix/commit/gnu/tests.scm?id=c0b726c2112e0e670dc134583b376541c0a3789a'>tests: Add &lt;system-test&gt; gexp compiler.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-01-23 11:26:55 +0100'>2020-01-23</span></td><td><a href='/guix/commit/gnu/tests.scm?id=f79313b3871ebe204c7b29fdb823666af06f48ff'>tests: Fix typo in comment.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
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<tr><td><span title='2019-09-23 23:41:19 +0200'>2019-09-23</span></td><td><a href='/guix/commit/gnu/tests.scm?id=7abd5997f41fec38ea1daa9099a9693062f10dbc'>repl, marionette: 'self-quoting?' matches keywords.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
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