lib/mpi: export mpi_rshift

A subsequent patch will make the crypto/dh's dh_is_pubkey_valid() to
calculate a safe-prime groups Q parameter from P: Q = (P - 1) / 2. For
implementing this, mpi_rshift() will be needed. Export it so that it's
accessible from crypto/dh.

Signed-off-by: Nicolai Stange <nstange@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>