Removed the reserved parameter from SDL_EnumerateDirectoryCallback

If someone needs to, say, include an SDL_Storage object, they can simply point userdata at a structure that includes the the storage and any other data needed in enumeration.

src/filesystem/posix/SDL_sysfsops.c

@@ -47,7 +47,7 @@ int SDL_SYS_FSenumerate(const char *fullpath, const char *dirname, SDL_Enumerate
         if ((SDL_strcmp(name, ".") == 0) || (SDL_strcmp(name, "..") == 0)) {
             continue;
         }
-        retval = cb(userdata, NULL, dirname, name);
+        retval = cb(userdata, dirname, name);
     }
 
     closedir(dir);
@@ -134,5 +134,5 @@ int SDL_SYS_FSstat(const char *fullpath, SDL_PathInfo *info)
     return 0;
 }
 
-#endif
+#endif // SDL_FSOPS_POSIX