common: Don't include intrin.h in a namespace

Fixes compilation with VS2017

Source/Core/Common/MathUtil.h

@@ -10,6 +10,10 @@
 
 #include "Common/CommonTypes.h"
 
+#ifdef _MSC_VER
+#include <intrin.h>
+#endif
+
 namespace MathUtil
 {
 template <typename T>
@@ -24,7 +28,6 @@ constexpr T SNANConstant()
 // will use __builtin_nans, which is improperly handled by the compiler and generates
 // a bad constant. Here we go back to the version MSVC used before the builtin.
 // TODO: Remove this and use numeric_limits directly whenever this bug is fixed.
-#include <intrin.h>
 
 template <>
 constexpr double SNANConstant()