spec: add explicit requires for -libs subpackage
    
    RPMDiff was complaining about this:
    
    ```
    Subpackage rpm-ostree on x86_64 consumes library
    librpmostree-1.so.1()(64bit) from subpackage rpm-ostree-libs but does
    not have explicit package version requirement.
    Please add Requires: rpm-ostree-libs = %{version}-%{release} to
    rpm-ostree in the specfile to avoid the need to test interoperability
    between the various combinations of old and new subpackages.
    ```
    
    Since we don't use any symbol versioning in rpm-ostree, this seems like
    a fair point. In practice, the matching -libs package should be
    available at the same time when composing/installing, though this
    protects us from manual `rpm` invocations as well.