Just like any other x_dependency.pri, it shall be included instead of
linking with png lib directly.

Change-Id: I698c8a8137f937d2e2c5f5dc9b9188b780c9e49a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>