agl-compositor RDEPENDS need updating

The runtime dependencies / RDEPENDS in the agl-compositor recipe are not complete, and a couple of dependencies only happen to work due to other packages being used in images.  One of these is the polkit dependency, which is added to the packagegroup-agl-graphical-weston package via a bbappend that is conditional on the presence of the meta-oe layer.  This is problematic since it breaks if that packagegroup is not used.  The other dependency is xkeyboard-config, which happens to get pulled in by the Qt dependency chain.  If agl-compositor is used in an image without Qt, then it fails to start.  These should ideally be resolved to ease building simple images with agl-compositor for e.g. virtualization hosts.