Add OpenBSD.

Index: runtime/druntime/src/rt/dmain2.d
--- runtime/druntime/src/rt/dmain2.d.orig
+++ runtime/druntime/src/rt/dmain2.d
@@ -44,6 +44,10 @@ version (NetBSD)
 {
     import core.stdc.fenv;
 }
+version (OpenBSD)
+{
+    import core.stdc.fenv;
+}
 version (DragonFlyBSD)
 {
     import core.stdc.fenv;
