package_manager: fix missing dependency on gnupg when signing deb package feeds

package_manager: fix missing dependency on gnupg when signing deb package feeds

When signing the deb package feed gpg tools are a soft requirement. If gnupg-native
is not declared a dependancy the version from hosttools is used. Unfortunately the
gpg-agent version from Ubuntu 16.04 on the autobuilders is incompatible with the package_index task
and fails during oe-selftest. Fix by making gnupg-native a dependency.

Fixes: 0b4231b5 "package_manager: sign DEB package feeds"
Reported-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Suggested-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Ferry Toth <ftoth@exalondelft.nl>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>