Fix if statement in catkin_libraries

[poggenhans]

Currently, when catkin_package(LIBRARIES ...) is called with an interface library that has more than one INTERFACE_LINK_LIBRARIES, catkin chokes on it with the error:

CMake Error at /opt/mrtros/share/catkin/cmake/catkin_libraries.cmake:140 (if):
  if given arguments:

    "library1" "library2"

  Unknown arguments specified

This PR fixes the syntax error that causes this problem. All followings commands seem to be able to handle multiple values in ${${lib}_interface_link_libraries}.

[dirk-thomas]

Thank you for the fix.