Fix struct vs. class mismatch in forward definitions

The warnings were being hidden by the use of -isystem to include
frameworks/native/include.

Bug: 31752268
Test: m -j
Change-Id: I46208cec43599f775f7e2cb22b39d19b0ea762e1
diff --git a/include/gui/BufferQueueProducer.h b/include/gui/BufferQueueProducer.h
index a75ed98..a48f45e 100644
--- a/include/gui/BufferQueueProducer.h
+++ b/include/gui/BufferQueueProducer.h
@@ -22,7 +22,7 @@
 
 namespace android {
 
-class BufferSlot;
+struct BufferSlot;
 
 class BufferQueueProducer : public BnGraphicBufferProducer,
                             private IBinder::DeathRecipient {