gnome-shell-calendar-server: add missing gtk.h include

6099a5dbc3 introduced this dependency
but didn't add it which lead to build failure with
-Werror=implicit-function-declaration

src/calendar-server/gnome-shell-calendar-server.c

@@ -34,6 +34,7 @@
 #include <unistd.h>
 
 #include <gio/gio.h>
+#include <gtk/gtk.h>
 
 #define HANDLE_LIBICAL_MEMORY
 #include <libecal/e-cal-client.h>