Describe the bug

• Restored temperature info for macOS #800
• This PR added host/include/smc.h and host/include/smc.c. These files are in the include sub-directory. This directory includes no go files, so Go modules does not include them when go mod vendor is used. Either the h & c files should be moved into the host directory, or a dummy go file should be added to include

To Reproduce
Go mod vendor gopsutil, then try to build:

# github.com/shirou/gopsutil/host
vendor/github.com/shirou/gopsutil/host/host_darwin_cgo.go:9:11: fatal error: 'include/smc.c' file not found
 #include "include/smc.c"
          ^~~~~~~~~~~~~~~
1 error generated.

Environment (please complete the following information):

• Windows: [paste the result of ver]
• Linux: [paste contents of /etc/os-release and the result of uname -a]
• Mac OS: [paste the result of sw_vers and uname -a
• FreeBSD: [paste the result of freebsd-version -k -r -u and uname -a]
• OpenBSD: [paste the result of uname -a]