Add USES=pkgconfig when GNOMEKEYRING is on.

This is actually required for the BUILD_KEYRING CMake option to have any
effect; in practice this means the GNOMEKEYRING option was not actually working
at all, and produced a package without the gnome-keyring plugin.

Bump PORTREVISION since a package with GNOMEKEYRING on was incorrect.

Reported by: fluffy