Fix compile error on FreeBSD
Fix compile error on FreeBSD

When compiling `20ecedd0b4c983bd7b88f97cd7a21461988a6c12` with GNU make (`gmake`) on FreeBSD 10.2-RELEASE, I get the following error:

```
collector/filesystem_bsd.go:60: non-bool mnt[i].f_flags & MNT_RDONLY (type C.uint64_t) used as if condition
Makefile.COMMON:85: recipe for target 'node_exporter' failed
gmake: *** [node_exporter] Error 2
```

This problem is fixed by this patch.

joehandzik/node_exporter,ema/node_exporter,bs-github/node_exporter,derekmarcotte/node_exporter,chosenken/node_exporter,cbstewart9393/node_exporter,siavashs/node_exporter,prometheus/node_exporter,chosenken/node_exporter,siavashs/node_exporter,cbstewart9393/node_exporter,Jean-Daniel/node_exporter,joehandzik/node_exporter,knweiss/node_exporter,knweiss/node_exporter,Jean-Daniel/node_exporter,ema/node_exporter,Jean-Daniel/node_exporter,mslocrian/flexswitch_exporter,knweiss/node_exporter,bs-github/node_exporter,derekmarcotte/node_exporter,prometheus/node_exporter,bs-github/node_exporter,joehandzik/node_exporter,bs-github/node_exporter,mslocrian/flexswitch_exporter,joehandzik/node_exporter,prometheus/node_exporter,chosenken/node_exporter,ema/node_exporter,knweiss/node_exporter,derekmarcotte/node_exporter,chosenken/node_exporter,Jean-Daniel/node_exporter,derekmarcotte/node_exporter,siavashs/node_exporter,siavashs/node_exporter,ema/node_exporter