Bug 1657706 - Forward-declare JSContext in nsrootidl.idl. r=nika

Right now we don't do that so stuff using implicit_jscontext compiles
just by chance.

Differential Revision: https://phabricator.services.mozilla.com/D86249

xpcom/base/nsrootidl.idl

@@ -17,6 +17,8 @@ typedef int64_t PRTime;
  */
 #include "nsStringFwd.h"
 
+struct JSContext;
+
 /*
  * Forward declaration of mozilla::dom::Promise
  */