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

Remove imports.h.
Remove mtypes.h.
Remove slang_compile.h.
Include glheader.h for GL symbols.
Include slang_vartable.h for slang_var_table symbol.

src/mesa/slang/slang_emit.h

@@ -25,11 +25,9 @@
 #ifndef SLANG_EMIT_H
 #define SLANG_EMIT_H
 
-
-#include "main/imports.h"
-#include "slang_compile.h"
+#include "main/glheader.h"
 #include "slang_ir.h"
-#include "main/mtypes.h"
+#include "slang_vartable.h"
 
 
 extern GLuint