Null plugin.soname.list for OpenVMS. b=112579. r=serge

modules/libpref/src/unix/openvms.js

@@ -55,4 +55,5 @@ pref("print.print_command", "print");
 pref("print.print_color", false);
 
 pref("browser.cache.disk.capacity", 4000);
+pref("plugin.soname.list", "");