Update `gcry_md_start_debug` to `gcry_md_debug`

Since libgcrypt 1.6.0, the deprecated message digest debug macros have
been removed. The `gcry_md_debug` should be used instead.

Reference: http://git.gnupg.org/cgi-bin/gitweb.cgi?p=libgcrypt.git;a=blob;f=NEWS;h=0150fdd6003172cc87d7f082a53a51f05644e206;hb=HEAD#l44