Forward declare struct Constants to fix a compile error.

This was breaking the Android build and didn't appear to be used in the code.

It was introduced with I0fe062ea826b73b2bdf28ad3ed6e72e51c576840

Change-Id: Ic2a00c3264263e5608903ee7764f9c8dd8fb2c2a
Reviewed-on: https://swiftshader-review.googlesource.com/3662
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>
diff --git a/src/Renderer/Renderer.hpp b/src/Renderer/Renderer.hpp
index e2008f3..61f8273 100644
--- a/src/Renderer/Renderer.hpp
+++ b/src/Renderer/Renderer.hpp
@@ -32,6 +32,7 @@
 	struct Task;
 	class Resource;
 	class Renderer;
+	struct Constants;
 
 	extern int batchSize;
 	extern int threadCount;