gtkimcontextsimple.c: Use X11_DATA_PREFIX only on X11
Unlike what commit d01ea18d says, X11 is not a requirement for Wayland, so a Wayland-only build is possible. We just use the same logic as other non-X11 platforms.
https://bugzilla.gnome.org/show_bug.cgi?id=784615
Signed-off-by: Quentin Glidic sardemff7+git@sardemff7.net
(cherry picked from commit f431e28c)