meson: move keyutils dependency check

Since there is not minimum version specified, and it's a test-only
dependency, it's fair to depend on a version that ships with a .pc I
suppose.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200828110734.1638685-8-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>