Fix compilation error w/o explicit default initialization of a const object.

The compilation error: error: default initialization of an object of const type 'const std::vector<MeshLib::Element *>' without a user-provided default constructor is produced by clang-3.6.0, which is correct for the current standard, but will be corrected later as described in http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#253

In gcc-5.1.0 this is only a warning.

See also this SO thread.