dependency on gmp.h.
(pgp_put_mpi): Condition on HAVE_LIBGMP.

* pgp.h: Don't include bignum.h, to make it possible to compile
the non-bignum parts of pgp-encode.c without bignum support. Needs
to be fixed properly before the pgp interface is advertised.

Rev: src/nettle/pgp-encode.c:1.8
Rev: src/nettle/pgp.h:1.6