bitbake.conf: Add gpg-agent as a host tool

If gpg is used, it will find the first gpg agent in the path, this
may lead to issues where gpg comes from the host, and the agent
comes from a gnupg-native due to package signing. The versions
being out of sync causes gpg to fail.

Signed-off-by: Daniel McGregor <daniel.mcgregor@vecima.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>