OvmfPkg/EmuVariableFvbRuntimeDxe: always format an auth varstore header
    
    In this patch, we extend commit d92eaabefbe0 ("OvmfPkg: simplify
    VARIABLE_STORE_HEADER generation", 2016-02-05) to
    EmuVariableFvbRuntimeDxe.
    
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>