Fix the JS standalone build. r=amazing_brokenness.

js/src/jsgc.cpp

@@ -1316,6 +1316,7 @@ ArenaLists::finalizeLater(JSContext *cx, AllocKind thingKind)
 #endif
 }
 
+#ifdef JS_THREADSAFE
 /*static*/ void
 ArenaLists::backgroundFinalize(JSContext *cx, ArenaHeader *listHead)
 {
@@ -1356,6 +1357,7 @@ ArenaLists::backgroundFinalize(JSContext *cx, ArenaHeader *listHead)
         lists->backgroundFinalizeState[thingKind] = BFS_DONE;
     }
 }
+#endif /* JS_THREADSAFE */
 
 void
 ArenaLists::finalizeObjects(JSContext *cx)

js/src/jsgc.h

@@ -1088,13 +1088,13 @@ struct ArenaLists {
 
 #ifdef JS_THREADSAFE
     static void backgroundFinalize(JSContext *cx, ArenaHeader *listHead);
+#endif
 
   private:
     inline void finalizeNow(JSContext *cx, AllocKind thingKind);
     inline void finalizeLater(JSContext *cx, AllocKind thingKind);
 
     inline void *allocateFromArena(JSContext *cx, AllocKind thingKind);
-#endif
 };
 
 /*