ArmVirtPkg/ArmVirtPrePiUniCoreRelocatable: list "PrePi.h" in INF file

ArmVirtPkg/ArmVirtPrePiUniCoreRelocatable: list "PrePi.h" in INF file

Among other things, the header file declares the ArchInitialize()
function, which is called from "PrePi.c", and implemented (dependent on
architecture) in "AArch64/ArchPrePi.c" and "Arm/ArchPrePi.c".

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Julien Grall <julien.grall@linaro.org>
Suggested-by: Michael Kinney <michael.d.kinney@intel.com>
Ref: http://mid.mail-archive.com/E92EE9817A31E24EB0585FDF735412F56327F7D3@ORSMSX113.amr.corp.intel.com
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>