Fix compiler warning -Wstring-plus-int

Fix compiler warning -Wstring-plus-int

Warning was as follows:

  src/tex2lyx/Parser.cpp:898:39: error: adding 'uint32_t' (aka 'unsigned int') to a string does not append to the string [-Werror,-Wstring-plus-int]
                  warning_message("ignoring a char: " + static_cast<uint32_t>(c));
                                  ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
  src/tex2lyx/Parser.cpp:898:39: note: use array indexing to silence this warning
                  warning_message("ignoring a char: " + static_cast<uint32_t>(c));
                                                      ^
                                  &                   [                         ]
  1 error generated.