mesa: prefix a bunch of #include lines with "main/".

This is another step toward removing a whole bunch of -I flags from
the cc commands.  Still need to address driver code...
diff --git a/src/mesa/shader/prog_execute.c b/src/mesa/shader/prog_execute.c
index b9b7712..e9957d3 100644
--- a/src/mesa/shader/prog_execute.c
+++ b/src/mesa/shader/prog_execute.c
@@ -35,9 +35,9 @@
  */
 
 
-#include "glheader.h"
-#include "colormac.h"
-#include "context.h"
+#include "main/glheader.h"
+#include "main/colormac.h"
+#include "main/context.h"
 #include "program.h"
 #include "prog_execute.h"
 #include "prog_instruction.h"