OvmfPkg/CsmSupportLib: list "LegacyRegion.h" in the INF file

OvmfPkg/CsmSupportLib: list "LegacyRegion.h" in the INF file

Among other things, the header file provides (extern) declarations for the
EFI_LEGACY_REGION2_PROTOCOL member functions that are defined in
"LegacyRegion.c". This way "mLegacyRegion2" can be initialized near the
top of "LegacyRegion.c", ahead of the member function definitions.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
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>