gnupg: option to include RSA support.

gnupg is compiled with --enable-minimal flag. This produces a binary that only
supports ElGamal and DSA public key algorithms.

RSA has been the default for `gpg --gen-key` since 2009, so it makes sense to
be able to build a gnupg binary that supports it.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>