Can not build with GOOS=freebsd

[lance6716]

$ GOOS=freebsd go build
# github.com/elastic/gosigar
./concrete_sigar.go:55:11: m.Get undefined (type Mem has no field or method Get)
./concrete_sigar.go:67:11: p.Get undefined (type HugeTLBPages has no field or method Get)
./concrete_sigar.go:79:12: fd.Get undefined (type FDUsage has no field or method Get)
./sigar_linux_common.go:70:4: undefined: parseCpuStat
./sigar_linux_common.go:88:4: undefined: parseCpuStat
./sigar_linux_common.go:106:18: undefined: getMountTableFileName

$ go version
go version go1.20.4 linux/amd64