Fix nsenter package on unsupported platforms.

Commit 4d1d6185ab added this nsenter_unsupported.go file in order for nsenter to be a valid (but empty, non-functional) Go package on unsupported platforms. However, on such platforms, Go still complains because there exists a .c file in this folder, but the package doesn't use cgo.
Fix that by importing "C" pseudo-package.

Signed-off-by: Dmitri Shuralyov <shurcooL@gmail.com>

nsenter/nsenter_unsupported.go

@@ -1,3 +1,5 @@
 // +build !linux !cgo
 
 package nsenter
+
+import "C"