build: Fix build with libxfce4ui master
Build error
xfce-keyboard-settings.c:846:18: error: call to undeclared function 'xfce_shortcut_conflict_dialog'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
846 | response = xfce_shortcut_conflict_dialog (GTK_WINDOW (dialog),
| ^
1 error generated.
Looks like a side effect of libxfce4ui@91898882