mapi: Adding missing string.h include.

File glapi_entrypoint.c calls memcpy() function, but does not include
string.h header. So compilation can fail at error: implicit declaration
of function 'memcpy'.

Signed-off-by: Jose Fonseca <jfonseca@vmware.com>

src/mapi/glapi/glapi_entrypoint.c

@@ -29,6 +29,8 @@
  */
 
 
+#include <string.h>
+
 #include "c11/threads.h"
 #include "glapi/glapi_priv.h"
 #include "u_execmem.h"