wet_shell_init() of fulscreen-shell has been missing to create a
screenshooter.
. adding a screenshooter_create() on wel_shell_init() for
fullscreen-shell.
. adding a dep_libexe_weston to meson.build to use the screenshooter
Signed-off-by: yj1231.heo <yj1231.heo@samsung.com>