OvmfPkg/OvmfPkg.dec: Add the new include folder for CSM header files

OvmfPkg/OvmfPkg.dec: Add the new include folder for CSM header files

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1811

The previous file-duplication commit has put all the CSM related header
files within folder: OvmfPkg/Csm/Inclue.

This commit will update the DEC file to add that folder under the
'[Include]' section.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: David Woodhouse <dwmw2@infradead.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>