[gjs/refactor-globals] Fix iwyu errors.

[Evan Welsh <ewlsh src gnome org>]

commit 75ad996ab752a3bb0ca08c6e9a0808f0fe119b81
Author: Evan Welsh <noreply evanwelsh com>
Date:   Sat Jul 18 01:32:06 2020 -0500

    Fix iwyu errors.

 gjs/context-private.h | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/gjs/context-private.h b/gjs/context-private.h
index 1b1e5562..c88537c0 100644
--- a/gjs/context-private.h
+++ b/gjs/context-private.h
@@ -55,6 +55,7 @@ template <class Key>
 struct DefaultHasher;
 }
 class GjsAtoms;
+class JSTracer;
 
 using JobQueueStorage =
     JS::GCVector<JS::Heap<JSObject*>, 0, js::SystemAllocPolicy>;