Fix gcc/MSVC compiler warnings

[StephenCWills]

gcc:

/home/runner/work/devilutionX/devilutionX/Source/items.cpp: In function ‘void devilution::UseItem(Player&, item_misc_id, SpellID, int)’:
/home/runner/work/devilutionX/devilutionX/Source/items.cpp:4274:35: warning: unused variable ‘spellLevel’ [-Wunused-variable]
 4274 |                         const int spellLevel = player.GetSpellLevel(spellID);
      |                                   ^~~~~~~~~~

Also, this doesn't show up in CI, probably because the build folder is cached, but libzt was generating a large number of warnings on my system because we weren't applying the -w flag to the zt_pic library.

---

MSVC:

Warning: D:\a\devilutionX\devilutionX\Source\engine\path.cpp(173): warning C4267: 'return': conversion from 'size_t' to 'int', possible loss of data

[StephenCWills]

Should we do something about the fmt compiler warnings in CI? I don't see those on my system.. maybe because I don't build with -DDEVILUTIONX_SYSTEM_LIBFMT=OFF.

EDIT: Actually, looking more closely, -DDEVILUTIONX_SYSTEM_LIBFMT=OFF wouldn't matter. Seems to me that the warning occurs because the compiler doesn't recognize the [[noreturn]] directive on the forward declaration for app_fatal() for some reason. Maybe a gcc version issue.