slider.h has compilation warning when building on arm

[RolandHughes]

In file included from /nanogui/include/nanogui/nanogui.h:33,
                 from /nanogui/src/python/python.h:7,
                 from /nanogui/src/python/nanovg.cpp:1:
/nanogui/include/nanogui/slider.h: In member function ‘std::pair<float, float> nanogui::Slider::range() const’:
/nanogui/include/nanogui/slider.h:34:43: note: parameter passing for argument of type ‘std::pair<float, float>’ when C++17 is enabled changed to match C++14 in GCC 10.1
   34 |     std::pair<float, float> range() const { return m_range; }
      |                                           ^

I know it is "upstream" problem, just wanted to pass the information along.