misc/cgo/test: skip Test18146 on DragonFly

Fails on builder for unknown reasons.

Fixes #18198.
Update #18146.

Change-Id: Iaa85826655eee57d86e0c73d06c930ef3f4647ec
Reviewed-on: https://go-review.googlesource.com/33906
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/test/issue18146.go

@@ -22,7 +22,7 @@ import (
 
 func test18146(t *testing.T) {
 	switch runtime.GOOS {
-	case "darwin", "openbsd":
+	case "darwin", "openbsd", "dragonfly":
 		t.Skip("skipping on %s; issue 18146", runtime.GOOS)
 	}