Add errors-on-non-Linux ToRootless implementation

`go test` complains about this tree because the package ends up with no
symbols on non-Linux platforms, e.g., Windows, even though this API is
never be called on non-Linux platforms, according to the compiler.

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>

util/rootless/specconv/specconv_nonlinux.go

@@ -0,0 +1,19 @@
+// +build !linux
+
+package specconv
+
+import (
+	"runtime"
+
+	"github.com/opencontainers/runtime-spec/specs-go"
+	"github.com/pkg/errors"
+)
+
+// ToRootless converts spec to be compatible with "rootless" runc.
+// * Remove /sys mount
+// * Remove cgroups
+//
+// See docs/rootless.md for the supported runc revision.
+func ToRootless(spec *specs.Spec) error {
+	return errors.Errorf("not implemented on on %s", runtime.GOOS)
+}