ecc-gost: Add missing include

* ecc-gost.c: Include "pubkey-internal.h".
--

Patch fixes compiler warning:

ecc-gost.c: In function '_gcry_ecc_gost_sign':
ecc-gost.c:95:11: warning: implicit declaration of function '_gcry_dsa_gen_k' [-Wimplicit-function-declaration]
           k = _gcry_dsa_gen_k (skey->E.n, GCRY_STRONG_RANDOM);
           ^

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
ecc: Refactor _gcry_ecc_eddsa_decodepoint.

* cipher/ecc-eddsa.c (_gcry_ecc_eddsa_decodepoint): Factor some code
out to ..
(_gcry_ecc_eddsa_recover_x): new.

Signed-off-by: Werner Koch <wk@gnupg.org>
ecc: Change algorithm for Ed25519 x recovery.

* cipher/ecc-eddsa.c (scanval): Add as temporary hack.
(_gcry_ecc_eddsa_recover_x): Use the algorithm from page 15 of the
paper.  Return an error code.
(_gcry_ecc_eddsa_decodepoint): Take care of the error code.
* mpi/mpi-mul.c (gcry_mpi_mulm): Use truncated division.

Signed-off-by: Werner Koch <wk@gnupg.org>