build: don't duplicate rule for weston.desktop

dist_wayland_session_DATA will distribute and install src/weston.desktop, so the
definition of wayland_session_DATA which also installs src/weston.desktop will
result in the file being installed twice and (rarely) cause install to fail.

Spotted and fix by Roy Li <rongqing.li@windriver.com>.
Signed-off-by: Ross Burton <ross.burton@intel.com>
Reviewed-By: Derek Foreman <derekf@osg.samsung.com>

Makefile.am

@@ -182,8 +182,7 @@ pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = src/weston.pc
 
 wayland_sessiondir = $(datadir)/wayland-sessions
-wayland_session_DATA = src/weston.desktop
+dist_wayland_session_DATA = src/weston.desktop
-dist_wayland_session_DATA = $(wayland_session_DATA)
 
 westonincludedir = $(includedir)/weston
 westoninclude_HEADERS =				\