Add Changelog entry

CHANGELOG.md

 # Master (will become release 2.8)
 
+- Remove the cmake check for `HAVE_MPROTECT` and also do not define this variable in the
+  `config.h` file. It is defined only inside the header `debugallocator.hh`.
+
 - Deprecate fallback implementations `Dune::Std::apply`, `Dune::Std::bool_constant`, and
   `Dune::Std::make_array` in favor of std c++ implementations.