Fix some kind of cast error on linux host

Hi !
This just happens when I started a ```go get``` :
```
# 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
```

I'm absolutely not a Go-guru, but do you think acceptable to do this change ?
It successfully fixed this casting issue !

Thx :)