Fix missing-declarations warnings all over the codebase

As caught by compiling with `-Wmissing-declarations`
by either making functions static, or declaring them in the header.

src/calls-provider.c

@@ -66,7 +66,7 @@ calls_provider_real_get_status (CallsProvider *self)
   return NULL;
 }
 
-GListModel *
+static GListModel *
 calls_provider_real_get_origins (CallsProvider *self)
 {
   return NULL;