compositor: Install weston-screenshooter in BINDIR

Previously weston-screenshooter was installed in LIBEXECDIR, but given
that now it can be invoked by the user whenever debug protocol is
enabled, let's intall it into BINDIR. This way, it can be invoked
without the need to modify PATH.

Signed-off-by: Marius Vlad <marius.vlad0@gmail.com>