add net_freebsd.go as placeholder.

host_freebsd.go

@@ -12,8 +12,8 @@ import (
 	"unsafe"
 )
 
-func HostInfo() (HostInfoStat, error) {
-	ret := HostInfoStat{}
+func HostInfo() (*HostInfoStat, error) {
+	ret := &HostInfoStat{}
 
 	hostname, err := os.Hostname()
 	ret.Hostname = hostname

net_freebsd.go

+// +build freebsd
+
+package gopsutil
+
+import (
+	"errors"
+)
+
+func NetIOCounters(pernic bool) ([]NetIOCountersStat, error) {
+	return nil, errors.New("not implemented yet")
+}