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-call-window.c

@@ -267,7 +267,7 @@ call_selector_child_activated_cb (GtkFlowBox      *box,
 }
 
 
-void
+static void
 add_call (CallsCallWindow *self,
           CallsCall       *call)
 {
@@ -288,7 +288,7 @@ add_call (CallsCallWindow *self,
   set_focus (self, display);
 }
 
-void
+static void
 remove_call (CallsCallWindow *self,
              CallsCall       *call,
              const gchar     *reason)

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;