Fix build with bazel on OSX

[tanyatik]

Following guide here:

bazel-contrib/rules_go#646 (comment)

Checked local build and seems to work.

[shirou]

Since I have not MacOSX now, I can't confirm. But it should work. Thank you for your contribution!

[shirou]

Sorry, This PR is reverted due to build error on linux. https://circleci.com/gh/shirou/gopsutil/357

go get -t -d -v ./...
can't load package: package github.com/shirou/gopsutil/host: C source files not allowed when not using cgo or SWIG: smc.c
package github.com/shirou/gopsutil/host: C source files not allowed when not using cgo or SWIG: smc.c

[tanyatik]

I created a new PR that should pass build on linux.
Added an empty file host_cgo.go , so that cgo is always used.

I also tried to build smc.c and smc.h into a library as suggested here bazel-contrib/rules_go#646 (comment) but it requires changes in BUILD.bazel which cannot be generated by gazelle.
This is quite hacky, so let me know if you have better idea on how to fix it.