x11: fix type mismatch warning in X.Display.get_window_property
x11: fix type mismatch warning in X.Display.get_window_property

Fixes bug 609561.

rvu95/vala,spidermonkdust/vala,rvu95/vala,jessevdk/vala,jessevdk/vala,tjol/vala-tjol,spidermonkdust/vala,severinh/vala,kamanashisroy/aroop,tjol/vala-tjol,Distrotech/vala,jessevdk/vala,frida/vala,GNOME/vala,severinh/vala,xdave/pvala,playya/vala,Distrotech/vala,frida/vala,playya/vala,frida/vala,rvu95/vala,kamanashisroy/aroop,flobrosch/vala-fixes,severinh/vala,flobrosch/vala-fixes,kamanashisroy/aroop,frida/vala,flobrosch/vala-fixes,tjol/vala-tjol,frida/vala,spidermonkdust/vala,GNOME/vala,GNOME/vala,playya/vala,Distrotech/vala,GNOME/vala