package/libpthsem: remove argp-standalone dependency

argp-standalone provide libargp.a and argp.h which are
never used in libpthsem.

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>