N3DS: Put `SDL_Log.txt` in the proper directory.

The homebrew "user" directory should be `/3ds/`.
To avoid ambiguity, `sdmc:` is specified.

src/SDL_log.c

@@ -488,7 +488,7 @@ SDL_LogOutput(void *userdata, int category, SDL_LogPriority priority,
 #elif defined(__3DS__)
     {
         FILE *pFile;
-        pFile = fopen ("/SDL_Log.txt", "a");
+        pFile = fopen("sdmc:/3ds/SDL_Log.txt", "a");
         fprintf(pFile, "%s: %s\n", SDL_priority_prefixes[priority], message);
         fclose(pFile);
     }