PixelRoutine: Fix warning about missing override.

I have no idea why this has suddenly started firing.

Bug: b/123933266
Change-Id: If94903e2b549bd628eb912569c1bb803bd576951
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27777
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>

src/Pipeline/PixelRoutine.hpp

diff --git a/src/Pipeline/PixelRoutine.hpp b/src/Pipeline/PixelRoutine.hpp
index 7b3bb4a..15c9307 100644
--- a/src/Pipeline/PixelRoutine.hpp
+++ b/src/Pipeline/PixelRoutine.hpp
@@ -48,7 +48,7 @@
 		virtual Bool alphaTest(Int cMask[4]) = 0;
 		virtual void rasterOperation(Pointer<Byte> cBuffer[4], Int &x, Int sMask[4], Int zMask[4], Int cMask[4]) = 0;
 
-		virtual void quad(Pointer<Byte> cBuffer[4], Pointer<Byte> &zBuffer, Pointer<Byte> &sBuffer, Int cMask[4], Int &x, Int &y);
+		void quad(Pointer<Byte> cBuffer[4], Pointer<Byte> &zBuffer, Pointer<Byte> &sBuffer, Int cMask[4], Int &x, Int &y) override;
 
 		void alphaTest(Int &aMask, Short4 &alpha);
 		void alphaToCoverage(Int cMask[4], Float4 &alpha);