tests/object_footprint: fix build error due to bad Kconfig specification

This was failing to build when the PWD was not that of the
tests/benchmarks/object_footprint because it was asking to load
$(PWD)/Kconfig.

Kconfig shall be specified with no directory location, as the file
will be imported relative to to Zephyr app's directory.

Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>