t/lib-gpg.sh: stop pretending to be a stand-alone script

It makes no sense to call `./lib-gpg.sh`. Therefore the hash-bang line is unnecessary. There are other similar instances in `t/`, but they are too far from the context of the enclosing patch series, so they will be addressed separately. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>