Fix a recent rustc nightly warning.
Fix a recent rustc nightly warning.

rtsuk/cairo,LordSentox/gtk,nicokoch/glib,daa84/gtk,rust-gnome/gtk,insanebits/gtk,gkoz/gtk-rs-cairo,mitaa/gtk,EPashkin/rust-gnome-gtk,insanebits/gtk,gkoz/rust-gnome-gtk,Susurrus/gtk,gkoz/gtk-rs-gtk,gkoz/rust-gnome-glib,rust-gnome/cairo,gkoz/gtk-rs-glib,rust-gnome/glib,daa84/gtk,drewet/gtk,gkoz/rust-gnome-cairo,rust-gnome/gtk,LordSentox/gtk,gkoz/gtk-rs-gtk,Susurrus/glib,EPashkin/rust-gnome-gtk,gkoz/gtk-rs-pango,gkoz/rust-gnome-gtk,drewet/gtk,EPashkin/rust-gnome-glib,Susurrus/gtk,mitaa/gtk