glsl: #undef THIS macro to fix MSVC build

THIS is a macro in one of the MSVC header files.  It's also a token
in the GLSL lexer.  This causes a compilation failure with MSVC.
This issue seems to be newly exposed after the recent mtypes.h removal
patches.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Neha Bhende <bhenden@vmware.com>

src/compiler/glsl/glsl_parser_extras.h

@@ -33,6 +33,11 @@
 #include <stdlib.h>
 #include "glsl_symbol_table.h"
 
+/* THIS is a macro defined somewhere deep in the Windows MSVC header files.
+ * Undefine it here to avoid collision with the lexer's THIS token.
+ */
+#undef THIS
+
 struct gl_context;
 
 struct glsl_switch_state {