gtkmountoperation-x11: unbreak compilation on OpenBSD.
gtkmountoperation-x11: unbreak compilation on OpenBSD.

Add missing header to prevent undeclared definitions.
Bug #659565

johne53/MB3Gtk-2,Distrotech/gtk,Distrotech/gtk,johne53/MB3Gtk-2,Distrotech/gtk,Unity-Technologies/gtk,johne53/MB3Gtk-2,Unity-Technologies/gtk,johne53/MB3Gtk-2,Unity-Technologies/gtk,Unity-Technologies/gtk,johne53/MB3Gtk-2,Distrotech/gtk,Distrotech/gtk,Unity-Technologies/gtk