Fix MSVC compilation: std::log was not accessible.

Eigen/src/Core/IO.h

diff --git a/Eigen/src/Core/IO.h b/Eigen/src/Core/IO.h
index 78a83b7..50bf93d 100644
--- a/Eigen/src/Core/IO.h
+++ b/Eigen/src/Core/IO.h
@@ -128,6 +128,7 @@
   static inline int run()
   {
     using std::ceil;
+    using std::log;
     return cast<RealScalar,int>(ceil(-log(NumTraits<RealScalar>::epsilon())/log(RealScalar(10))));
   }
 };