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
diff --git a/src/mesa/shader/program.c b/src/mesa/shader/program.c
index 13d6df1..02e23aa 100644
--- a/src/mesa/shader/program.c
+++ b/src/mesa/shader/program.c
@@ -29,9 +29,9 @@
  */
 
 
-#include "glheader.h"
-#include "context.h"
-#include "hash.h"
+#include "main/glheader.h"
+#include "main/context.h"
+#include "main/hash.h"
 #include "program.h"
 #include "prog_cache.h"
 #include "prog_parameter.h"