Make xcb QPA plugin link on FreeBSD

dlopen, etc. are defined in libc on FreeBSD.

Change-Id: Iad47113ec77029f8e300e4f08f454d70b3070b5a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>