build: fallback to __sync_synchronize()

If we're running on a GCC older than 4.9, then we won't have the
stdatomic.h available. We can just use a full barrier instead using
__sync_synchronize() to get the same effect, albeit slower.

src/libsysprof-capture/mapped-ring-buffer.c

@@ -22,7 +22,6 @@
 
 #include "config.h"
 
-#include <stdatomic.h>
 #include <sys/mman.h>
 #include <sys/types.h>
 #include <unistd.h>