ArmVirtPkg/PlatformPeiLib: make PcdLib dependency explicit in .INF

ArmVirtPkg/PlatformPeiLib: make PcdLib dependency explicit in .INF

We currently include PcdLib.h in PlatformPeiLib, without declaring
this dependency in its .INF description. Since all the PCDs we use
resolve to fixed type in practice, this does not really matter at
the moment, but since we will be adding dynamic PCD references in
a subsequent patch, let's make the PcdLib dependency explicit, so
that its dispatch is guaranteed to be ordered correctly with respect
to the provider of the dynamic PCD PPI.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2560
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>