xlib_libpthread-stubs: Rename package to libpthread-stubs

Signed-off-by: Bernd Kuhls <berndkuhls@hotmail.com>
Signed-Off-By: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Spenser Gilliland <spenser@gillilanding.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>