Fix some kind of cast error on linux host

Fixing this error on specific Linux host :
```
# github.com/cloudfoundry/gosigar
../../cloudfoundry/gosigar/sigar_linux.go:93: cannot use sysinfo.Totalswap (type uint32) as type uint64 in assignment
../../cloudfoundry/gosigar/sigar_linux.go:94: cannot use sysinfo.Freeswap (type uint32) as type uint64 in assignment
```