Fix compilation on OpenBSD

Fix compilation on OpenBSD

  CC       src/libcollectdclient/libcollectdclient_la-server.lo
In file included from src/libcollectdclient/server.c:37:
/usr/include/net/if.h:121: error: field 'ifi_lastchange' has incomplete type
/usr/include/net/if.h:369: error: field 'ifru_addr' has incomplete type
/usr/include/net/if.h:370: error: field 'ifru_dstaddr' has incomplete type
/usr/include/net/if.h:371: error: field 'ifru_broadaddr' has incomplete type
/usr/include/net/if.h:399: error: field 'ifrau_addr' has incomplete type
/usr/include/net/if.h:405: error: field 'ifra_dstaddr' has incomplete type
/usr/include/net/if.h:407: error: field 'ifra_mask' has incomplete type
/usr/include/net/if.h:450: error: field 'addr' has incomplete type
/usr/include/net/if.h:451: error: field 'dstaddr' has incomplete type
/usr/include/net/if.h:457: error: expected specifier-qualifier-list before 'sa_family_t'
In file included from /usr/include/net/if.h:466,
                 from src/libcollectdclient/server.c:37:
/usr/include/net/if_arp.h:79: error: field 'arp_pa' has incomplete type
/usr/include/net/if_arp.h:80: error: field 'arp_ha' has incomplete type
*** Error 1 in . (Makefile:6017 'src/libcollectdclient/libcollectdclient_la-server.lo': @echo "  CC      " src/libcollectdclient/libcollectd...)
*** Error 1 in /home/ruben/src/collectd (Makefile:4188 'all')