rtt_roscomm: Boost header generation broken with genmsg 0.4.22

ros/genmsg#32 has broken typekit generation as `${${package}_MESSAGE_FILES}` variable contains relative paths now. The current Ubuntu repositories serve genmsg 0.4.22 including that patch since today.

We have to check if this can be considered a bug in genmsg or if there is a simple way to get the absolute path in a generic way.

Possible solution (not tested yet):
```cmake
if(NOT IS_ABSOLUTE "${FILE}")
  set(FILE "${${package}_DIR}/${FILE}")
endif()
```