bpfilter: ignore binary files

net/bpfilter/bpfilter_umh is a binary file generated when bpfilter is
enabled, add it to .gitignore to avoid committing it.

Fixes: d2ba09c1 ("net: add skeleton of bpfilter kernel module")
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>