homedir: add cgo or osusergo buildtag constraints for unix

This is to ensure that users of the homedir package cannot
compile statically (CGO_ENABLED=0) without also setting the osusergo
build tag.

Signed-off-by: Tibor Vass <tibor@docker.com>

pkg/homedir/homedir_unix.go

@@ -1,4 +1,4 @@
-// +build !windows
+// +build !windows,cgo !windows,osusergo
 
 package homedir // import "github.com/docker/docker/pkg/homedir"