Fix warning

Fix warning

This was:
../../master/src/mathed/InsetMathBox.cpp:116:20: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<std::__cxx11::basic_string<wchar_t> >::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
  116 |  for (int i = 0; i < parts.size(); i += 2) {
      |                  ~~^~~~~~~~~~~~~~
../../master/src/mathed/InsetMathBox.cpp:120:20: warning: comparison of integer expressions of different signedness: ‘std::vector<std::__cxx11::basic_string<wchar_t> >::size_type’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]