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 <mchqwerty@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

Makefile.am

@@ -158,7 +158,7 @@ pkgconfig_DATA = src/weston.pc
 
 wayland_sessiondir = $(datadir)/wayland-sessions
 wayland_session_DATA = src/weston.desktop
-dist_wayland_session_DATA += $(wayland_session_DATA)
+dist_wayland_session_DATA = $(wayland_session_DATA)
 
 westonincludedir = $(includedir)/weston
 westoninclude_HEADERS =				\