OvmfPkg/BaseMemEncryptSevLib: list "X64/VirtualMemory.h" in the INF file

OvmfPkg/BaseMemEncryptSevLib: list "X64/VirtualMemory.h" in the INF file

Among other things, the header file declares the
InternalMemEncryptSevSetMemoryDecrypted() and
InternalMemEncryptSevSetMemoryEncrypted() functions. The functions are
called from "X64/MemEncryptSevLib.c", and defined in
"X64/VirtualMemory.c".

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
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>