glsl: Include missing headers in slang_compiler_operation.h.

Include glheader.h for GL symbols.
Include slang_compile_variable.h for slang_variable_scope symbol.
Include slang_utility.h for slang_atom symbol.

src/mesa/slang/slang_compile_operation.h

@@ -26,6 +26,10 @@
 #define SLANG_COMPILE_OPERATION_H
 
 
+#include "main/glheader.h"
+#include "slang_compile_variable.h"
+#include "slang_utility.h"
+
 /**
  * Types of slang operations.
  * These are the types of the AST (abstract syntax tree) nodes.