Fix build with Apple Clang

[Yadunund]

The default compiler on macOS devices is Apple Clang which throws an error for undefined std::to_string() call. This PR adds the <string> include to fix compilation.

/Users/yadunund/ws_rmf/install/rmf_utils/include/rmf_utils/Modular.hpp:64:22: error: no member named 'to_string' in namespace 'std'
              + std::to_string(Window));

[luca-della-vedova]

Trivial "include-what-you-use"

[luca-della-vedova]

First commit not signed, can you sign off / force push?

[mxgrey]

I've used admin powers to override the DCO requirement for such a trivial and obviously necessary change.