Avoid unused parameter warning in BOOST_FUSION_DEFINE_STRUCT_INLINE

Submitting this patch as a pull request, see also https://svn.boost.org/trac/boost/ticket/10679.

Original description:
Expanding the BOOST_FUSION_DEFINE_STRUCT_INLINE macro leads to unused parameter warnings about 'it1' and 'it2', even when the boost include path is marked with -isystem.

The attached patch should avoid the warning.