glsl: Clean up header file inclusion in slang_mem.h.

slang_mem.h
Remove imports.h.
Include glheader.h for GL symbols.

slang_label.c
Include imports.h now that slang_mem.c does not include it.

src/mesa/slang/slang_label.c

@@ -7,6 +7,7 @@
  */
 
 
+#include "main/imports.h"
 #include "main/mtypes.h"
 #include "program/prog_instruction.h"
 #include "slang_label.h"

src/mesa/slang/slang_mem.h

@@ -27,7 +27,7 @@
 #define SLANG_MEM_H
 
 
-#include "main/imports.h"
+#include "main/glheader.h"
 
 
 typedef struct slang_mempool_ slang_mempool;