Bug 1579323 - Reenable sandbox by default on OpenBSD r=froydnj

This was broken by bug 1375863

--HG--
extra : histedit_source : 4f4c20c63d0a6337888eb6b21c451d025c72eb1a

old-configure.in

@@ -2197,7 +2197,7 @@ MOZ_ARG_DISABLE_BOOL(sandbox,
     MOZ_SANDBOX=1)
 
 case "$OS_TARGET" in
-WINNT|Darwin)
+WINNT|Darwin|OpenBSD)
 ;;
 Linux)
     case $CPU_ARCH in
@@ -2210,7 +2210,7 @@ Linux)
     esac
     ;;
 *)
-    # Only enable the sandbox by default on Linux, macOS, and Windows
+    # Only enable the sandbox by default on Linux, OpenBSD, macOS, and Windows
     MOZ_SANDBOX=
 ;;
 esac