OvmfPkg: add TimeBaseLib resolution for OvmfXen
    
    Commit 55ee36b0c490
    ("EmbeddedPkg/RealTimeClockRuntimeDxe: Use helper functions from TimeBaseLib")
    added a TimeBaseLib dependency to RealTimeClockRuntimeDxe, which now breaks
    build of OvmfXen.dsc.
    
    Add a resolution for EmbeddedPkg/Library/TimeBaseLib/TimeBaseLib.inf.
    
    Cc: Laszlo Ersek <lersek@redhat.com>
    Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
    Cc: Anthony Perard <anthony.perard@citrix.com>
    Cc: Julien Grall <julien@xen.org>
    Signed-off-by: Leif Lindholm <leif@nuviainc.com>
    Reviewed-by: Laszlo Ersek <lersek@redhat.com>