stack_trace.cc: fix #elif check for OS_MACOSX

Fix '#elif with no expression' add defined() to check.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>

port/stack_trace.cc

@@ -71,7 +71,7 @@ void PrintStackTraceLine(const char* symbol, void* frame) {
 
   fprintf(stderr, "\n");
 }
-#elif OS_MACOSX
+#elif defined(OS_MACOSX)
 
 void PrintStackTraceLine(const char* symbol, void* frame) {
   static int pid = getpid();