meson: Fix -Dbuiltin_immodules=backend not including waylandgtk
As before in modules/input/Makefile.am:
if INCLUDE_IM_WAYLAND
STATIC_WAYLANDGTK_MODULE = libstatic-im-waylandgtk.la
Currently using -Dbuiltin_immodules=backend
causes /usr/lib64/gtk-3.0/3.0.0/immodules/im-waylandgtk.so
to be installed.
Edited by Chris Mayo