treewide: mark things static and const

Mark some structures, arrays static/const at various places.
In some cases this prevents unnecessary initialization
when a function is entered.

All in all, the text segments across all shared
libraries are reduced by about 2 KiB. However,
the total size increases by about 2 KiB as well.

src/modules/module-protocol-simple.c

@@ -451,7 +451,7 @@ static void on_core_proxy_destroy(void *data)
 	client_cleanup(client);
 }
 
-static struct pw_proxy_events core_proxy_events = {
+static const struct pw_proxy_events core_proxy_events = {
 	PW_VERSION_CORE_EVENTS,
 	.destroy = on_core_proxy_destroy,
 };