mesa: Prefix main includes with dir to avoid conflicts.

Some of the headers in src/mesa/main have pretty common names which
easily conflict with third-party code, e.g. config.h

src/mesa/shader/prog_statevars.c

@@ -29,12 +29,12 @@
  */
 
 
-#include "glheader.h"
-#include "context.h"
-#include "hash.h"
-#include "imports.h"
-#include "macros.h"
-#include "mtypes.h"
+#include "main/glheader.h"
+#include "main/context.h"
+#include "main/hash.h"
+#include "main/imports.h"
+#include "main/macros.h"
+#include "main/mtypes.h"
 #include "prog_statevars.h"
 #include "prog_parameter.h"