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='2017-12-27 21:04:37 +0100'>2017-12-27</span></td><td><a href='/guix/commit/gnu/packages/javascript.scm?id=94b779b6a4d0bbd0693e293d2f6f38afa6e8a6c3'>gnu: mujs: Update to 1.0.2.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
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<tr><td><span title='2017-08-04 11:23:53 +0200'>2017-08-04</span></td><td><a href='/guix/commit/gnu/packages/javascript.scm?id=0b0aa76fa84b421e6c7c127984a24211439f6146'>gnu: Add js-es5-shim.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2017-08-04 11:23:53 +0200'>2017-08-04</span></td><td><a href='/guix/commit/gnu/packages/javascript.scm?id=4f65067bde7e886c42a9549eb0e16706b43f8af7'>gnu: Add js-selectize.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2017-08-04 11:23:53 +0200'>2017-08-04</span></td><td><a href='/guix/commit/gnu/packages/javascript.scm?id=c4abc94d2b3106e336573ec4644fb09e2b6c6368'>gnu: Add js-datatables.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2017-08-04 11:23:53 +0200'>2017-08-04</span></td><td><a href='/guix/commit/gnu/packages/javascript.scm?id=a6bbb2a0d864c68356baa7dc94b801cbb769a117'>gnu: Add js-highlight.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2017-08-04 11:23:52 +0200'>2017-08-04</span></td><td><a href='/guix/commit/gnu/packages/javascript.scm?id=3f48403aafc3cd6e50c178fb15b6251a40969c07'>gnu: Add js-strftime.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
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<tr><td><span title='2017-08-04 11:23:52 +0200'>2017-08-04</span></td><td><a href='/guix/commit/gnu/packages/javascript.scm?id=ed69963b03d71928f8c926ed62e5d6f5e78d9b2a'>gnu: Add js-html5shiv.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2017-06-23 08:14:41 +0200'>2017-06-23</span></td><td><a href='/guix/commit/gnu/packages/javascript.scm?id=b37b48f8192678c1c0f72a62d3ad33c19da55d14'>gnu: Add js-respond.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2017-06-05 19:16:06 +0530'>2017-06-05</span></td><td><a href='/guix/commit/gnu/packages/javascript.scm?id=a3d3b7a4e137ce977852342d75125279cf0ca41f'>gnu: Add js-mathjax.</a><span class='msg-avail'>...</span></td><td>Arun Isaac</td></tr>