cpp: Make GpgME::Data::toKeys really const
cpp: Make GpgME::Data::toKeys really const

* lang/cpp/src/data.cpp (GpgME::Data::toKeys): Rewind afterards.

--
This fixes unexpected behavior that the seek pointer is changed
after calling the const toKeys.

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme