Make PGP macros global

This commit moves the macros PGP_SIGNATURE and PGP_MESSAGE to gpg-interface.h to
make them publicly reachable, which is necessary for RFC3161 time-stamping.

When verifying an RFC3161 time-stamp signature, a SHA-1 hash has to be generated
over the git object excluding a possible PGP signature, which makes it necessary
to detect the beginning of this PGP signature.

Signed-off-by: Anton Würfel <anton.wuerfel@fau.de>
Signed-off-by: Phillip Raffeck <phillip.raffeck@fau.de>