build: move argp-standalone into contrib/ directory

Change-Id: Iedcddf95c3577da644c0aebbb297b04c93f1b6fe
BUG: 1081274
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/7352
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>