Fix getopt handling on architectures where char is unsigned

[AdrianBunk]

gcc says:

src/main.cpp: In function ‘int main(int, char**)’:
src/main.cpp:41:85: warning: comparison is always true due to limited range of data type [-Wtype-limits]
   41 |     while ((opt = getopt_long(argc, argv, "m:n:g:e:q:c:l:r:d:h", options, nullptr)) != -1) {
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~