bindings: cxx: fix building with clang's libc++

bindings: cxx: fix building with clang's libc++

This fixes the following error with clang 11 with libc++:

 line.cpp:248:39: error: implicit instantiation of undefined template 'std::__1::array<gpiod_line_event, 16>'
         ::std::array<::gpiod_line_event, 16> event_buf;
                                              ^
 /nix/store/vk8ynr4hj2a8w7g1b9m8wknzj39iiv4x-libc++-11.0.1/include/c++/v1/__tuple:219:64: note: template is declared here
 template <class _Tp, size_t _Size> struct _LIBCPP_TEMPLATE_VIS array;

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>