Enable vectorized erf<double>(x) for SSE and AVX, which was accidentally removed in merge request 1750.

Eigen/src/Core/arch/AVX/PacketMath.h

diff --git a/Eigen/src/Core/arch/AVX/PacketMath.h b/Eigen/src/Core/arch/AVX/PacketMath.h
index 7e4f054..c29523a 100644
--- a/Eigen/src/Core/arch/AVX/PacketMath.h
+++ b/Eigen/src/Core/arch/AVX/PacketMath.h
@@ -145,6 +145,7 @@
 #endif
     HasTanh = EIGEN_FAST_MATH,
     HasLog = 1,
+    HasErf = 1,
     HasErfc = 1,
     HasExp = 1,
     HasSqrt = 1,

Eigen/src/Core/arch/SSE/PacketMath.h

diff --git a/Eigen/src/Core/arch/SSE/PacketMath.h b/Eigen/src/Core/arch/SSE/PacketMath.h
index f294009..77f0cfa 100644
--- a/Eigen/src/Core/arch/SSE/PacketMath.h
+++ b/Eigen/src/Core/arch/SSE/PacketMath.h
@@ -217,6 +217,7 @@
     HasCos = EIGEN_FAST_MATH,
     HasTanh = EIGEN_FAST_MATH,
     HasLog = 1,
+    HasErf = EIGEN_FAST_MATH,
     HasErfc = EIGEN_FAST_MATH,
     HasExp = 1,
     HasSqrt = 1,