Add missing header

Resolves the following build-time warning:

[..]/libqmenumodel/src/gtk/gtksimpleactionobserver.c: In function ‘gtk_simple_action_observer_register_action’:
[..]/libqmenumodel/src/gtk/gtksimpleactionobserver.c:147:7: warning: implicit declaration of function ‘gtk_action_observable_register_observer’ [-Wimplicit-function-declaration]
  147 |       gtk_action_observable_register_observer (self->observable, self->action_name, GTK_ACTION_OBSERVER (self));
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[..]/libqmenumodel/src/gtk/gtksimpleactionobserver.c: In function ‘gtk_simple_action_observer_unregister_action’:
[..]/libqmenumodel/src/gtk/gtksimpleactionobserver.c:156:5: warning: implicit declaration of function ‘gtk_action_observable_unregister_observer’ [-Wimplicit-function-declaration]
  156 |     gtk_action_observable_unregister_observer(self->observable, self->action_name, GTK_ACTION_OBSERVER (self));
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1 files changed, 1 insertions, 0 deletions

diff --git a/libqmenumodel/src/gtk/gtksimpleactionobserver.c b/libqmenumodel/src/gtk/gtksimpleactionobserver.c
index 054b82f..14d37e0 100644
--- a/libqmenumodel/src/gtk/gtksimpleactionobserver.c
+++ b/libqmenumodel/src/gtk/gtksimpleactionobserver.c
@@ -18,6 +18,7 @@
  */
 
 #include "gtksimpleactionobserver.h"
+#include "gtkactionobservable.h"
 
 typedef GObjectClass GtkSimpleActionObserverClass;