UefiCpuPkg/MpInitLib: Add missing explicit PcdLib dependency

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2632



Both PEI and DXE instances of the MpInitLib are using PcdLib APIs, but
none of them list the dependency of the PcdLib in INF & header files.

This commit will explicitly add such dependency in .H and .INF files.

Test done:
Library level build pass for VS2015x86 tool chain

Cc: Eric Dong <eric.dong@intel.com>
Signed-off-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>