Fix a clang 3.8.0 warning in pflogd.c:

contrib/pf/pflogd/pflogd.c:769:8: error: logical not is only applied to the left hand side of this comparison [-Werror,-Wlogical-not-parentheses]

if (!if_exists(interface) == -1) {
    ^                     ~~

The if_exists() function does not return -1, and even if it did, it
would not be the correct way to check. Just ditch the == -1 instead.

Obtained from: OpenBSD's pflogd.c 1.49
MFC after: 3 days