Don't mark a non-constexpr-able function as constexpr

It can't be constexpr because it needs write access to an std::array, and
because of std::sqrt.

alc/alu.cpp

@@ -573,7 +573,7 @@ struct RotatorCoeffs {
     };
     std::array<CoeffValues,CalcRotatorSize<MaxAmbiOrder>()> mCoeffs{};
 
-    constexpr RotatorCoeffs()
+    RotatorCoeffs()
     {
         auto coeffs = mCoeffs.begin();