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>