Platform/Sgi: Fix missing FdtLib instance issue
Include the FdtLib path to fix a build issue coming on Arm/SgiPkg with
PlatformStandaloneMm2.
Fixes the build breakage introduced by 9ad168c9e0:
StandaloneMmPkg: Include libfdt in the StMM
Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>