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...

src/mesa/tnl/t_vertex.c

@@ -25,9 +25,9 @@
  *    Keith Whitwell <keithw@tungstengraphics.com>
  */
 
-#include "glheader.h"
-#include "context.h"
-#include "colormac.h"
+#include "main/glheader.h"
+#include "main/context.h"
+#include "main/colormac.h"
 
 #include "t_context.h"
 #include "t_vertex.h"