gpg-interface: use size_t for signature buffer size

gpg-interface: use size_t for signature buffer size

Even though our object sizes (from which these buffers would
come) are typically "unsigned long", this is something we'd
like to eventually fix (since it's only 32-bits even on
64-bit Windows). It makes more sense to use size_t when
taking an in-memory buffer.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Ben Toews <mastahyeti@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>