Makefile: fix assigning to dist_wayland_session_DATA
Makefile: fix assigning to dist_wayland_session_DATA

dist_wayland_session_DATA is not set anywhere before, so using +=
results in:

error: dist_wayland_session_DATA must be set with '=' before using '+='

Signed-off-by: Marek Chalupa <a05df156a69ad59faa9af9b666ef78009594c145@gmail.com>
Reviewed-by: Bryce Harrington <bdb14cfc1ab8d24e13de84adc40b41ae60c13bd1@osg.samsung.com>

Tarnyko/weston-xdg_surface_present,udoprog/weston,giucam/weston,xorgy/weston,kwm81/weston,Fantu/compositor-spice,krezovic/weston,sir-murray/weston,udoprog/weston,krezovic/weston,mchalupa/weston,mchalupa/weston,Tarnyko/weston-xdg_surface_present,giucam/weston,jonnylamb/weston,xorgy/weston,Fantu/compositor-spice,eyolfson/weston,kwm81/weston,sir-murray/weston,eyolfson/weston,Fantu/compositor-spice,jonnylamb/weston,Gnurou/weston,Gnurou/weston