Fix no previous prototype for function error

src/lazy/SkDiscardableMemoryPool.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkDiscardableMemory.h"
 #include "SkDiscardableMemoryPool.h"
+#include "SkImageGenerator.h"
 #include "SkLazyPtr.h"
 #include "SkTInternalLList.h"
 #include "SkThread.h"
 
 // Note:
 // A PoolDiscardableMemory is memory that is counted in a pool.
 // A DiscardableMemoryPool is a pool of PoolDiscardableMemorys.
 
 namespace {
 
@@ skipping 242 matching lines @@
 SkDiscardableMemoryPool* SkGetGlobalDiscardableMemoryPool() {
     SK_DECLARE_STATIC_LAZY_PTR(SkDiscardableMemoryPool, global, create_global_pool);
     return global.get();
 }
 
 // defined in SkImageGenerator.h
 void SkPurgeGlobalDiscardableMemoryPool() {
     SkGetGlobalDiscardableMemoryPool()->dumpPool();
 }
 ////////////////////////////////////////////////////////////////////////////////