OvmfPkg: resolve PcdLib for all PEIMs individually
    
    Currently the default (module type independent) PcdLib resolution is to
    BasePcdLibNull.inf, which is inherited by all PEIMs. In the next patch,
    we'll flip the PEIM default resolution to PeiPcdLib.inf, but in order to
    keep that patch both correct and simple to review, we should spell out the
    Null resolution for those two PEIMs (ReportStatusCodeRouterPei and
    StatusCodeHandlerPei) that are now the only ones that don't specify an
    explicit resolution.
    
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>