mesa: Include mtypes.h in files that use gl_context struct.

Directly include mtypes.h if a file uses a gl_context struct. This
allows future removal of headers that are not strictly necessary but
indirectly include mtypes.h for a file.

src/mesa/main/texcompress_s3tc.c

@@ -39,6 +39,7 @@
 #include "dlopen.h"
 #include "image.h"
 #include "macros.h"
+#include "mtypes.h"
 #include "texcompress.h"
 #include "texcompress_s3tc.h"
 #include "texstore.h"