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

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

The header file declares the functions LegacyRegionInit(),
LegacyInterruptInstall() and LegacyBiosPlatformInstall(). They are defined
in "LegacyRegion.c", "LegacyInterrupt.c", and "LegacyPlatform.c",
respectively, and are all called from CsmSupportLibConstructor() in
"CsmSupportLib.c".

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>