selftests: drv-net: add rss_api to the Makefile

I missed adding rss_api.py to the Makefile. The NIPA Makefile
checking script was scanning for shell scripts only, so it
didn't flag it either.

Fixes: 4d13c6c4 ("selftests: drv-net: test RSS Netlink notifications")
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250712012005.4010263-1-kuba@kernel.org


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