selftests: net: update .gitignore with missing test

Fixes: 192dc405f308 ("selftests: net: add tcp_mmap program")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/.gitignore

@@ -7,3 +7,4 @@ reuseport_bpf_cpu
 reuseport_bpf_numa
 reuseport_dualstack
 reuseaddr_conflict
+tcp_mmap