Fix Wunused-parameter warning

Test: presubmit
Change-Id: Iee153445dbbbde6652702e213546dd794ac93895
diff --git a/Eigen/src/Core/arch/NEON/GeneralBlockPanelKernel.h b/Eigen/src/Core/arch/NEON/GeneralBlockPanelKernel.h
index 3481f33..ee80899 100644
--- a/Eigen/src/Core/arch/NEON/GeneralBlockPanelKernel.h
+++ b/Eigen/src/Core/arch/NEON/GeneralBlockPanelKernel.h
@@ -24,7 +24,7 @@
 
   template <typename LaneIdType>
   EIGEN_STRONG_INLINE void madd(const Packet4f& a, const Packet4f& b,
-                                Packet4f& c, Packet4f& tmp,
+                                Packet4f& c, [[maybe_unused]] Packet4f& tmp,
                                 const LaneIdType&) const {
     acc(a, b, c);
   }