spa: fix initializer for old GCC

Older gcc versions seem to require the members to appear in the
designated initializer in the order they are in the definition of
the struct when compiling C++.

otherwise compilation fails with:

../spa/plugins/aec/aec-webrtc.cpp:167:1: sorry, unimplemented:
non-trivial designated initializers not supported
 };
 ^

spa/plugins/aec/aec-webrtc.cpp

@@ -161,6 +161,8 @@ static int webrtc_run(void *data, const float *rec[], const float *play[], float
 }
 
 static struct spa_audio_aec_methods impl_aec = {
+    .version = 0,
+	.add_listener = NULL,
 	.init = webrtc_init,
 	.run = webrtc_run,
 };