meson: link editor with gobject-2.0

editor.c calls g_clear_object(), so it should link to gobject directly instead
of relying on pangocairo pulling it in in its pkg-config.

Fixes: https://gitlab.freedesktop.org/wayland/weston/issues/211



Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>