selftests net: fix kselftest net fatal error

The incorrect path is causing the following error when trying to run net
kselftests:

In file included from bpf/nat6to4.c:43:
../../../lib/bpf/bpf_helpers.h:11:10: fatal error: 'bpf_helper_defs.h' file not found
         ^~~~~~~~~~~~~~~~~~~
1 error generated.

Fixes: cf67838c

 ("selftests net: fix bpf build error")
Signed-off-by: Coleman Dietsch <dietschc@csp.edu>
Link: https://lore.kernel.org/r/20220628174744.7908-1-dietschc@csp.edu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>