You can use pointer to this structure without including the header that contains its definition, if it is possible, of course.
Also, it should be possible to #define class to smth else just before including this header and then #undef'ing it. If this C library comes in binary form, it shouldn't cause any problems since memory layout won't be affected; if it comes in source code form, you can always compile it with -Dclass=smth_else on the command line and reference this foul structure field from your C++ code accordingly.