Include ModelicaUtilities.h as C

Otherwise the MSVC linker complains with
```
msgpack-modelica.obj : error LNK2019: unresolved external symbol "void __cdecl ModelicaFormatError(char const *,...)" (?ModelicaFormatError@@YAXPBDZZ) referenced in function _msgpack_modelica_sbuffer_to_file
msgpack-modelica.obj : error LNK2019: unresolved external symbol "void __cdecl ModelicaError(char const *)" (?ModelicaError@@YAXPBD@Z) referenced in function _msgpack_modelica_unpack_int
msgpack-modelica.obj : error LNK2019: unresolved external symbol "char * __cdecl ModelicaAllocateString(unsigned int)" (?ModelicaAllocateString@@YAPADI@Z) referenced in function _msgpack_modelica_unpack_string
```