selftests: bpf: notification about privilege required to run test_kmod.sh testing script

The test_kmod.sh script require root privilege for the successful
execution of the test.

This patch is to notify the user about the privilege the script
demands for the successful execution of the test.

Signed-off-by: Jeffrin Jose T (Rajagiri SET) <ahiliation@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>