gtk+3: disable wayland without opengl

gtk+3: disable wayland without opengl

Wayland backend requires epoxy so it will pull in a requirement for opengl
While doing some tests in building old stuff like matchbox-terminal with
vte 0.76, I encountered an issue with building matchbox-terminal without
opengl in DISTRO_FEATURES.

Explicitly stating that the Wayland backend requires opengl fixes this problem
and allows matchbox-terminal to be used without.

Signed-off-by: Markus Volk <f_l_k@t-online.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>