glcpp: Rename the variable used to enable debugging.

The -p option we now use when calling bison means that this variable will be
named glcpp_parser_debug not yydebug. This was not caught when the -p option
was added because this variable isn't used in the code as committed. (I prefer
the declaration to remain since it allows a developer to easily find this
variable name to enable debugging.)

src/glsl/glcpp/glcpp.c

@@ -30,7 +30,7 @@
 #include "main/mtypes.h"
 #include "main/shaderobj.h"
 
-extern int yydebug;
+extern int glcpp_parser_debug;
 
 void
 _mesa_reference_shader(struct gl_context *ctx, struct gl_shader **ptr,