agent: Fix for suggested Libgcrypt use.

* agent/divert-scd.c (divert_pkdecrypt): Skip a flags parameter.

The libgcrypt docs say that a "flags" parameter should always be used
in the input of pkdecrypt. Thus we should allow that parameter also
when parsing an s-expression to figure out the algorithm for use with
scdaemon.

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