core: forward declare nm_settings_plugin_get_type() at file scope in "nm-settings-storage.h"

lgtm.com warns about function declarations inside blocks.
*sigh*. I think it's well understood what this code means, and it is not
done by accident. Still, let's make the tool happy in this case.

src/core/settings/nm-settings-storage.h

@@ -48,11 +48,12 @@ GType nm_settings_storage_get_type(void);
 NMSettingsStorage *
 nm_settings_storage_new(struct _NMSettingsPlugin *plugin, const char *uuid, const char *filename);
 
+/* forward declare so we don't have to include "nm-settings-plugin.h" here. */
+GType nm_settings_plugin_get_type(void);
+
 static inline struct _NMSettingsPlugin *
 nm_settings_storage_get_plugin(const NMSettingsStorage *self)
 {
-    GType nm_settings_plugin_get_type(void);
-
     g_return_val_if_fail(NM_IS_SETTINGS_STORAGE(self), NULL);
 
     nm_assert(G_TYPE_CHECK_INSTANCE_TYPE(self->_plugin, nm_settings_plugin_get_type()));