Unbreak VC++ build.

llvm-svn: 32113
diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index 1b355b6..359766d 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -20,6 +20,7 @@
 #include "llvm/Support/GetElementPtrTypeIterator.h"
 #include "llvm/Support/MathExtras.h"
 #include <cerrno>
+#include <cmath>
 using namespace llvm;
 
 //===----------------------------------------------------------------------===//