Unbreak VC++ build.

llvm-svn: 32113
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 9528cb4..1dcf9fd 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -29,6 +29,7 @@
 #include <iostream>
 #include <set>
 #include <algorithm>
+#include <cmath>
 using namespace llvm;
 
 /// makeVTList - Return an instance of the SDVTList struct initialized with the