bindings: cxx: examples: add missing include

This patch adds proper include file to fix FTBFS issue in
gpiod_cxx_tests.cpp, please see below error.

gpiod_cxx_tests.cpp:29:19: note: 'std::function' is defined in header '<functional>'; did you forget to '#include <functional>'?
gpiod_cxx_tests.cpp:26:1:
+#include <functional>

Signed-off-by: SZ Lin (林上智) <sz.lin@moxa.com>
[Bartosz: tweaked the commit message]
Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
1 file changed