Lock warning when adding a controller

[edumeneses]

There is a warning when adding a game controller to the system while puara-controller is already running:

Assertion failure at SDL_AssertJoysticksLocked, triggered 1 time:
  'SDL_JoysticksLocked()'

We (most likely) need to manually lock access to the game controller while it is unavailable.

This error pops a window at Linux-based systems and requires manual interaction (abort or ignore)