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

@@ -487,10 +487,10 @@ SDL_LogOutput(void *userdata, int category, SDL_LogPriority priority,
     }
 #elif defined(__3DS__)
     {
-        FILE*        pFile;
-        pFile = fopen ("/SDL_Log.txt", "a");
+        FILE *pFile;
+        pFile = fopen("sdmc:/3ds/SDL_Log.txt", "a");
         fprintf(pFile, "%s: %s\n", SDL_priority_prefixes[priority], message);
-        fclose (pFile);
+        fclose(pFile);
     }
 #endif
 #if HAVE_STDIO_H && \