MathUtil: Mark IntLog2 as constexpr

Mostly everything else was already marked as constexpr, and the only
function IntLog2 calls (CountLeadingZeros) is already constexpr.

Source/Core/Common/MathUtil.h

@@ -191,7 +191,7 @@ private:
 float MathFloatVectorSum(const std::vector<float>&);
 
 // Rounds down. 0 -> undefined
-inline int IntLog2(u64 val)
+constexpr int IntLog2(u64 val)
 {
   return 63 - Common::CountLeadingZeros(val);
 }