Makefile: fix systemd-notify plugin CFLAGS

Makefile: fix systemd-notify plugin CFLAGS

SYSTEMD_DAEMON_LIBS does not belong in CFLAGS, but SYSTEMD_DAEMON_CFLAGS
does. Fix that.

Add the missing COMPOSITOR_CFLAGS. Otherwise compiling the plugin will
use the system wayland-server.h when it should be using the one pointed
to by pkg-config.

The latter fixes the build for me, as my system libwayland-server is
older than what Weston and this plugin require, and the correct version
is only found in my install $prefix.

Cc: Egor Starkov <egor.starkov@ge.com>
Cc: Eugen Friedrich <efriedrich@de.adit-jv.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>