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