Suppress unused variable warning in ip_dummynet.c

With clang 15, the following -Werror warning is produced:

sys/netpfil/ipfw/ip_dummynet.c:802:6: error: variable 'n' set but not used [-Werror,-Wunused-but-set-variable]
        int n = 0; /* only for stats */
            ^

Here, 'n' is a variable that is only used when debugging. Mark the
variable as potentially unused, to suppress the warning.

MFC after: 3 days

(cherry picked from commit 5c329f0a4d32cec54250ec2d3f61455e85137a6f)