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.

plugins/ofono/calls-ofono-provider.h

@@ -25,10 +25,11 @@
 #ifndef CALLS_OFONO_PROVIDER_H__
 #define CALLS_OFONO_PROVIDER_H__
 
+#include "calls-provider.h"
+
 #include <glib-object.h>
 #include <gio/gio.h>
-
-#include "calls-provider.h"
+#include <libpeas/peas.h>
 
 G_BEGIN_DECLS
 
@@ -36,6 +37,8 @@ G_BEGIN_DECLS
 
 G_DECLARE_FINAL_TYPE (CallsOfonoProvider, calls_ofono_provider, CALLS, OFONO_PROVIDER, CallsProvider)
 
+void              peas_register_types                       (PeasObjectModule *module);
+
 G_END_DECLS
 
 #endif /* CALLS_OFONO_PROVIDER_H__ */