Fix breakage in GPU build.

Eigen/src/Core/arch/Default/GenericPacketMathFunctionsFwd.h

diff --git a/Eigen/src/Core/arch/Default/GenericPacketMathFunctionsFwd.h b/Eigen/src/Core/arch/Default/GenericPacketMathFunctionsFwd.h
index ea4c8d0..d9e495f 100644
--- a/Eigen/src/Core/arch/Default/GenericPacketMathFunctionsFwd.h
+++ b/Eigen/src/Core/arch/Default/GenericPacketMathFunctionsFwd.h
@@ -68,7 +68,7 @@
 
 /** \internal \returns atan(x) */
 template <typename Packet>
-Packet generic_patan(const Packet& x);
+EIGEN_DEFINE_FUNCTION_ALLOWING_MULTIPLE_DEFINITIONS Packet generic_patan(const Packet& x);
 
 /** \internal \returns exp(x) for single precision float */
 template <typename Packet>