Fix clang-tidy errors

Created by: armandas

We are using clang-tidy for static analysis and we get the following error

error: The left expression of the compound assignment is an uninitialized value. The computed value will also be garbage [clang-analyzer-core.uninitialized.Assign,-warnings-as-errors]
    _flags &= KEY_IS_OWNED;
           ^

in this function:

  void setType(uint8_t t) {
    _flags &= KEY_IS_OWNED;
    _flags |= t;
  }

This could be resolved by adding the SYSTEM keyword to target_include_directories, but I'm aware that GCC prior to version 9 wraps all system headers in extern "C", breaking the C++ libraries.

Maybe we could add an option to specify if the SYSTEM keyword should be added? Any other ideas?