glthread: make marshal functions for glBegin/End attribs non-static

for a future commit

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20824>

src/mapi/glapi/gen/marshal_XML.py

@@ -88,4 +88,6 @@ class marshal_function(gl_XML.gl_function):
         return 'async'
 
     def marshal_is_static(self):
-        return self.marshal_flavor() != 'custom' and self.name[0:8] != 'Internal'
+        return (self.marshal_flavor() != 'custom' and
+                self.name[0:8] != 'Internal' and
+                self.exec_flavor != 'beginend')