graphics: Include stddef.h for size_t.

graphics.h uses size_t, but doesn't pull in a declaration. This results
in compile errors if code that includes graphics.h (such as
hardware/hardware.h) doesn't happen to have a size_t declaration from
elsewhere.

Change-Id: I0f3882c3dce20425697d25366d49930b2251ef7f

include/system/graphics.h

@@ -17,6 +17,7 @@
 #ifndef SYSTEM_CORE_INCLUDE_ANDROID_GRAPHICS_H
 #define SYSTEM_CORE_INCLUDE_ANDROID_GRAPHICS_H
 
+#include <stddef.h>
 #include <stdint.h>
 
 #ifdef __cplusplus