build: Fix includes according to latest IWYU config

gi/boxed.h

@@ -44,6 +44,7 @@
 
 class BoxedPrototype;
 class BoxedInstance;
+class JSTracer;
 namespace JS {
 class CallArgs;
 }

gi/object.cpp

@@ -75,6 +75,8 @@
 #include "gjs/mem-private.h"
 #include "util/log.h"
 
+class JSTracer;
+
 /* This is a trick to print out the sizes of the structs at compile time, in
  * an error message. */
 // template <int s> struct Measure;

gi/object.h

@@ -53,6 +53,7 @@
 #include "util/log.h"
 
 class GjsAtoms;
+class JSTracer;
 namespace JS {
 class CallArgs;
 }

gi/wrapperutils.h

@@ -54,6 +54,7 @@
 
 struct JSFunctionSpec;
 struct JSPropertySpec;
+class JSTracer;
 
 GJS_JSAPI_RETURN_CONVENTION
 bool gjs_wrapper_to_string_func(JSContext* cx, JSObject* this_obj,

gjs/atoms.h

@@ -33,6 +33,8 @@
 
 #include "gjs/macros.h"
 
+class JSTracer;
+
 // clang-format off
 #define FOR_EACH_ATOM(macro) \
     macro(code, "code") \

gjs/importer.cpp

@@ -34,6 +34,7 @@
 #include <vector>   // for vector
 
 #include <gio/gio.h>
+#include <glib-object.h>
 #include <glib.h>
 
 #include <js/CallArgs.h>