Add glibc-langpack-en BuildRequires for en_US.UTF-8 locale
    
    Fedora 30 will remove glibc-all-langpacks from the default buildroot¹.
    Ensure en_US.UTF-8 locale is present for the test suite.  Avoid C.UTF-8
    until it is more widely used and tested.
    
    ¹ https://fedoraproject.org/wiki/Changes/Remove_glibc-langpacks-all_from_buildroot