fix compile error with gcc

panda/src/wgldisplay/wglGraphicsStateGuardian.cxx

@@ -585,10 +585,9 @@ get_extra_extensions() {
 void *wglGraphicsStateGuardian::
 do_get_extension_func(const char *prefix, const char *name) {
   string fullname = string(prefix) + string(name);
-  return wglGetProcAddress(fullname.c_str());
+  return (void*) wglGetProcAddress(fullname.c_str());
 }
 
-
 ////////////////////////////////////////////////////////////////////
 //     Function: wglGraphicsStateGuardian::make_context
 //       Access: Private