fvp: Use the right implem. of plat_report_exception() in BL3-2

On FVP, the file 'plat/fvp/aarch64/plat_helpers.S' contains an
FVP-specific implementation of the function 'plat_report_exception()',
which is meant to override the default implementation. However, this
file was not included into the BL3-2 image, meaning it was still
using the default implementation. This patch fixes the FVP makefile
to compile this file in.