[ros2] add missing set header

[mikaelarguedas]

WIthout this it fails to compile on gcc 9.2 with the following:

/tmp/demo_deps_ws/src/image_common/image_transport/src/publisher.cpp:111:8: error: ‘set’ is not a member of ‘std’
  111 |   std::set<std::string> blacklist;
      |        ^~~
/tmp/demo_deps_ws/src/image_common/image_transport/src/publisher.cpp:44:1: note: ‘std::set’ is defined in header ‘<set>’; did you forget to ‘#include <set>’?

cc @mjcarroll

Signed-off-by: Mikael Arguedas mikael.arguedas@gmail.com

[galou]

Shouldn't this be back-ported to the dashing branch?