module-loopback: don't use 0 for custom source output message id

Message id 0 is PA_SOURCE_OUTPUT_MESSAGE_GET_LATENCY. So, every time PulseAudio
sent PA_SOURCE_OUTPUT_MESSAGE_GET_LATENCY message to the loopback source output,
it actually hit the SOURCE_OUTPUT_MESSAGE_LATENCY_SNAPSHOT handler instead. As a
result, the SOURCE_OUTPUT_MESSAGE_LATENCY_SNAPSHOT handler was called when not
intended, the default PA_SOURCE_OUTPUT_MESSAGE_GET_LATENCY handler was not called
at all, and the latency was thus evaluated incorrectly.

Reported-by: Georg Chini <georg@chini.tk>
Signed-off-by: Alexander E. Patrakov <patrakov@gmail.com>

src/modules/module-loopback.c

@@ -126,7 +126,7 @@ enum {
 };
 
 enum {
-    SOURCE_OUTPUT_MESSAGE_LATENCY_SNAPSHOT
+    SOURCE_OUTPUT_MESSAGE_LATENCY_SNAPSHOT = PA_SOURCE_OUTPUT_MESSAGE_MAX,
 };
 
 static void enable_adjust_timer(struct userdata *u, bool enable);