Silence GCC warnings about void*-to-function-pointer casts

GCC warns on casting `void*` to function pointers. This is because the C
standard makes a clear distinction between pointers to objects (`void*`)
and pointers to functions (function pointers), and does not specify
anything related to being able to cast object pointers to function
pointers.

The warning message is factually wrong, though - it states that it is
forbidden by ISO C, when in fact it is not, and is actually just
unspecified.

We can't get rid of the cast entirely, because we need the explicit cast
(the C standard _does_ mandate you need an explicit cast when converting
between object pointers and function pointers), and at the end of the
day, this is simply how `SDL_LoadFunction()` works (and more
importantly, how `dlsym()` works), so we can't get rid of it, and we
have no reason to anyways since it means we don't have a hard runtime
dependency on Steam (unlike some other games) and casting `void*` to
function pointers always behaves well on every single platform we ship
on that supports Steam.

Unfortunately, this warning seems to be a part of -Wpedantic, and
there's no way to disable this warning specifically without disabling
-Wpedantic. Luckily, I've found a workaround - just cast to `intptr_t`
before casting to the function pointer. Hopefully the compiler doesn't
get smarter in the future and this ends up breaking or anything...

desktop_version/src/SteamNetwork.c

@@ -86,7 +86,7 @@ int32_t STEAM_init(void)
     }
 
 #define FOREACH_FUNC(rettype, name, params) \
-    name = (rettype (*) params) SDL_LoadFunction(libHandle, #name); \
+    name = (rettype (*) params) (intptr_t) SDL_LoadFunction(libHandle, #name); \
     if (!name) \
     { \
         vlog_error(STEAM_LIBRARY " symbol " #name " not found!"); \