fullscreen-shell : fullscreen-shell doesn't support the screenshooter

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>