Add IWYU private pragmas to internal headers.
diff --git a/Eigen/src/Core/arch/GPU/PacketMath.h b/Eigen/src/Core/arch/GPU/PacketMath.h
index a04c563..5c959ed 100644
--- a/Eigen/src/Core/arch/GPU/PacketMath.h
+++ b/Eigen/src/Core/arch/GPU/PacketMath.h
@@ -10,6 +10,7 @@
 #ifndef EIGEN_PACKET_MATH_GPU_H
 #define EIGEN_PACKET_MATH_GPU_H
 
+// IWYU pragma: private
 #include "../../InternalHeaderCheck.h"
 
 namespace Eigen {