build: Fix compile failure in backtrace_posix.cc

Fix compile failure in backtrace_posix.c on AIX

Fixes: #7539
PR-URL: #7544
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Brian White <mscdex@mscdex.net>

src/backtrace_posix.cc

@@ -4,7 +4,7 @@
 #include <features.h>
 #endif
 
-#if defined(__linux__) && !defined(__GLIBC__)
+#if defined(__linux__) && !defined(__GLIBC__) || defined(_AIX)
 #define HAVE_EXECINFO_H 0
 #else
 #define HAVE_EXECINFO_H 1