Add workaround for missing fl_method_xxx_response_get_type() symbols

Using the following macros:

- FL_METHOD_SUCCESS_RESPONSE()
- FL_METHOD_ERROR_RESPONSE()
- FL_METHOD_NOT_IMPLEMENTED_RESPONSE()

Would result to linking errors:

    /usr/bin/ld: foo_plugin.so: undefined reference to `fl_method_success_response_get_type'
    /usr/bin/ld: foo_plugin.so: undefined reference to `fl_method_error_response_get_type'
    /usr/bin/ld: foo_plugin.so: undefined reference to `fl_method_not_implemented_response_get_type'

See also commit 6e378cf.

Author: jpnurmi

shell/platform/linux/fl_method_response.cc

@@ -26,8 +26,11 @@ struct _FlMethodNotImplementedResponse {
   FlMethodResponse parent_instance;
 };
 
-// Added here to stop the compiler from optimising this function away.
+// Added here to stop the compiler from optimising these functions away.
 G_MODULE_EXPORT GType fl_method_response_get_type();
+G_MODULE_EXPORT GType fl_method_success_response_get_type();
+G_MODULE_EXPORT GType fl_method_error_response_get_type();
+G_MODULE_EXPORT GType fl_method_not_implemented_response_get_type();
 
 G_DEFINE_TYPE(FlMethodResponse, fl_method_response, G_TYPE_OBJECT)
 G_DEFINE_TYPE(FlMethodSuccessResponse,