mesa: include c11/threads.h in mtypes.h

Let's directly include c11/threads.h instead of relying on glapi.h
to provide it.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>

src/mesa/main/mtypes.h

@@ -36,6 +36,7 @@
 
 #include <stdint.h>             /* uint32_t */
 #include <stdbool.h>
+#include "c11/threads.h"
 
 #include "main/glheader.h"
 #include "main/config.h"