add baxter_msgs build dependency

[codebot]

i followed the instructions here: https://github.com/davetcoleman/baxter/tree/groovy-devel and found that I needed to add this build dependency in order to avoid what appeared to be a build-time race condition where it tried to build baxter_pick_place before it built baxter_msgs. probably depends on the number of cores on the build box, etc.

[davetcoleman]

Thanks!