Fix no previous prototype for function error

BUG=skia:2720
R=scroggo@google.com, caryclark@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/363983002

src/lazy/SkDiscardableMemoryPool.cpp

@@ -7,6 +7,7 @@
 
 #include "SkDiscardableMemory.h"
 #include "SkDiscardableMemoryPool.h"
+#include "SkImageGenerator.h"
 #include "SkLazyPtr.h"
 #include "SkTInternalLList.h"
 #include "SkThread.h"