BranchProbability.cpp - add missing implicit cmath header dependency. NFCI.

Noticed while investigating if we can remove an unnecessary MathExtras.h include from SmallVector.h (necessary for gcc builds but not MSVC)

lib/Support/BranchProbability.cpp

@@ -16,6 +16,7 @@
 #include "llvm/Support/Format.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cassert>
+#include <cmath>
 
 using namespace llvm;