spec: silence krb5 pkgconf errors in %krb5_base_version
    
    Send stderr of pkgconf to /dev/null rather than printing the following
    error text while parsing the spec file:
    
        Package krb5 was not found in the pkg-config search path.
        Perhaps you should add the directory containing `krb5.pc'
        to the PKG_CONFIG_PATH environment variable
        Package 'krb5', required by 'virtual:world', not found
    
    `BuildRequires: pkgconfig(krb5)` ensures this won't happen when running
    a real build.  It simply avoids 4 lines of needless error output when
    running something like `fedpkg prep`.
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>