Add missing 'BuildRequires: gcc'
    
    A C compiler is necessary to build Toolbx [1].  GCC is being pulled in
    by the other BuildRequires, but it's good to explicitly list it since
    GCC isn't part of the default buildroot since Fedora 29 [2] and will
    prevent the compiler from unexpectedly changing to Clang.
    
    There's no need to do a build just for this.
    
    [1] Upstream commit c8aaed52c547e24e
        https://github.com/containers/toolbox/commit/c8aaed52c547e24e
        https://github.com/containers/toolbox/pull/923
    
    [2] https://fedoraproject.org/wiki/Changes/Remove_GCC_from_BuildRoot