reorder includes to compile on OpenBSD

From getifaddr(3) manual:
"If both <net/if.h> and <ifaddrs.h> are being
included, <net/if.h> must be included before <ifaddrs.h>"

http://www.openbsd.org/cgi-bin/man.cgi?query=getifaddrs&apropos=0&sektion=0&manpath=OpenBSD+Current&arch=i386&format=html
Merge pull request #4119

2364b11 reorder includes to compile on OpenBSD (Fabian Raetz)