tr_bsp: fix a signedness comparison problem

[illwieckz]

Noticed in a CI log while looking for something else:

src/engine/renderer/tr_bsp.cpp: In function ‘std::vector<std::__cxx11::basic_string<char> > R_LoadExternalLightmaps(const char*)’:
src/engine/renderer/tr_bsp.cpp:472:22: warning: comparison of integer expressions of different signedness: ‘int’ and ‘long long unsigned int’ [-Wsign-compare]
  472 |   for ( int i = 0; i < ARRAY_LEN( extensions ); i++ )