Don't include <gcrypt.h> when not necessary

[Ludovic Courtès]

Hello,

For the record I just pushed this patch, which allows compilation
without libgcrypt:

  http://git.savannah.gnu.org/cgit/gnutls.git/commit/?h=gnutls_2_12_x&id=da7563294b63d006a15c8bdcbb3d6d4d87b72558

Thanks,
Ludo’.