selftests/net: Add log.txt and tools to .gitignore

Update .gitignore to untrack tools directory and log.txt. "tools" is
generated in "selftests/net/Makefile" and log.txt is generated in
"selftests/net/gro.sh" when executing run_all_tests.

Signed-off-by: Anh Tuan Phan <tuananhlfc@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>