Ignore ARDOUR_DATA_PATH env variable on windows

The variable is unset (and unused), windows_search_path()
is used on that platform.

This prevents the "ARDOUR_DATA_PATH not set in environment"
message from being printed.

libs/ardour/filesystem_paths.cc

@@ -290,13 +290,14 @@ ardour_data_search_path ()
 		search_path += user_config_directory();
 #ifdef PLATFORM_WINDOWS
 		search_path += windows_search_path ();
-#endif
+#else
 		std::string s = Glib::getenv("ARDOUR_DATA_PATH");
 		if (s.empty()) {
 			std::cerr << _("ARDOUR_DATA_PATH not set in environment\n");
 		} else {
 			search_path += Searchpath (s);
 		}
+#endif
 	}
 
 	return search_path;