build-unit-test-docker: Add function2

This is a drop-in replacement for std::function that is planned to be
adopted by sdeventplus for storing callback functions. The major benefit
here is the unique_function type that can store functions bound with
move-only types like unique_ptr.

Change-Id: Id3abedf1c1adb237d2ddcdc20c9720a300d203fe
Signed-off-by: William A. Kennington III <wak@google.com>
1 file changed