Move libdecnumber to the static package
    
    The Fedora Packaging Guidelines [1] require that static libraries are
    packaged separately in order to be able to track build requirements.
    This commit is also moving libdecnumber's header files to this new
    package as they have no value in the devel package.
    
    [1] https://docs.fedoraproject.org/en-US/packaging-guidelines/#packaging-static-libraries