Add safe_strerror() to leak detective whitelist

While the thread specific strerror buffer gets cleaned up for
worker threads during their termination, the main thread itself,
and so its strerror buffer, is still alive during leak reports.

src/libstrongswan/utils/leak_detective.c

@@ -188,6 +188,7 @@ static void uninstall_hooks()
 char *whitelist[] = {
 	/* backtraces, including own */
 	"backtrace_create",
+	"safe_strerror",
 	/* pthread stuff */
 	"pthread_create",
 	"pthread_setspecific",