meson: declare keyutils dependency

Rename the variable to be more explicit. A further clean-up patch will
move the actual to dependency check to meson entirely.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200828110734.1638685-4-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>