Revert "[common.math] minor fix"

This reverts commit 8b201f5ee86fa08202ea9410f290562a46ff7eec because it was an anti-fix.

dune/stuff/common/math.hh

@@ -33,7 +33,7 @@ namespace boost {
 namespace math {
 //! isinf specialization for Dune::StraightenBlockVector
 template <class BlockVectorImp, class DofImp>
-inline bool isinf(const Dune::StraightenBlockVector<BlockVectorImp, DofImp>& x)
+inline bool isinf(const Dune::Fem::StraightenBlockVector<BlockVectorImp, DofImp>& x)
 {
   for (size_t i = 0; i < x.size(); ++i) {
     if (std::isinf(x[i]))