selftests: netdevsim: add fib_notifications to Makefile

Commit 19d36d29 ("selftests: netdevsim: Add fib_notifications test")
added the test but didn't include it in the Makefile.

Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20241029192603.509295-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>