[SIMD] Allow custom cmath-overloads to be found

Lukas Reneltrequested to merge
lukas.renelt/dune-istl:fix/fix-use-of-cmath-functions into master

Directly invoking std::f() prevents custom overloads for e.g. SIMD-types of being found. Fix this by calling using std::f; f().

Edited by Lukas Renelt

Merge request reports