cipher: Hardening ElGamal by introducing exponent blinding too.

* cipher/elgamal.c (do_encrypt): Also do exponent blinding.

Base blinding had been introduced with USE_BLINDING. This patch add
exponent blinding as well to mitigate side-channel attack on mpi_powm.

• GnuPG-bug-id: T5328
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>