Compile netdev on dragonfly (#314)

* Compile netdev on dragonfly

* Only run netdev bsd test on bsd

* Update README.md

README.md

@@ -27,7 +27,7 @@ filesystem | Exposes filesystem statistics, such as disk space used. | FreeBSD,
 loadavg | Exposes load average. | Darwin, Dragonfly, FreeBSD, Linux, NetBSD, OpenBSD, Solaris
 mdadm | Exposes statistics about devices in `/proc/mdstat` (does nothing if no `/proc/mdstat` present). | Linux
 meminfo | Exposes memory statistics. | FreeBSD, Linux
-netdev | Exposes network interface statistics such as bytes transferred. | FreeBSD, Linux, OpenBSD
+netdev | Exposes network interface statistics such as bytes transferred. | Dragonfly, FreeBSD, Linux, OpenBSD
 netstat | Exposes network statistics from `/proc/net/netstat`. This is the same information as `netstat -s`. | Linux
 stat | Exposes various statistics from `/proc/stat`. This includes CPU usage, boot time, forks and interrupts. | Linux
 textfile | Exposes statistics read from local disk. The `--collector.textfile.directory` flag must be set. | _any_

collector/netdev_bsd.go

@@ -12,6 +12,7 @@
 // limitations under the License.
 
 // +build !nonetdev
+// +build freebsd dragonfly
 
 package collector
 

collector/netdev_bsd_test.go

@@ -11,6 +11,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// +build !nonetdev
+// +build freebsd dragonfly
+
 package collector
 
 import "testing"

collector/netdev_common.go

@@ -12,7 +12,7 @@
 // limitations under the License.
 
 // +build !nonetdev
-// +build linux freebsd openbsd
+// +build linux freebsd openbsd dragonfly
 
 package collector