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

Move mtypes.h and prog_instruction.h to slang_label.c.
Remove imports.h.
Include glheader.h from GL symbols.

src/mesa/slang/slang_label.c

@@ -7,6 +7,8 @@
  */
 
 
+#include "main/mtypes.h"
+#include "program/prog_instruction.h"
 #include "slang_label.h"
 #include "slang_mem.h"
 

src/mesa/slang/slang_label.h

@@ -1,10 +1,9 @@
 #ifndef SLANG_LABEL_H
 #define SLANG_LABEL_H 1
 
-#include "main/imports.h"
-#include "main/mtypes.h"
-#include "program/prog_instruction.h"
+#include "main/glheader.h"
 
+struct gl_program;
 
 struct slang_label_
 {