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>
n>
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<div class='path'>path: <a href='/guix/log/?id=084e675ac884a27123e1198a08114d35f438f06e'>root</a>/<a href='/guix/log/scripts?id=084e675ac884a27123e1198a08114d35f438f06e'>scripts</a>/<a href='/guix/log/scripts/guix.in?id=084e675ac884a27123e1198a08114d35f438f06e'>guix.in</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/scripts/guix.in?id=084e675ac884a27123e1198a08114d35f438f06e&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-12-11 19:06:53 +0100'>2020-12-11</span></td><td><a href='/guix/commit/scripts/guix.in?id=fd247542215fbd10f28980f3d5255014b150e95a'>build: 'script/guix' uses our own 'guile' executable.</a><span class='msg-avail'>...<span class='msg-tooltip'>* Makefile.am (do_subst): Substitute @abs_top_builddir@.
* scripts/guix.in: Use it.
</span></span></td><td>Ludovic Courtès</td></tr>