Merge pull request #20 from suve/add-SDL_mutex

Add SDL_mutex.inc (v3.2.12)

Author: Free-Pascal-meets-SDL-Website

units/SDL3.pas

@@ -126,6 +126,7 @@ interface
 {$I SDL_process.inc}                      // 3.2.0
 {$I SDL_storage.inc}                      // 3.2.0
 {$I SDL_tray.inc}                         // 3.2.12
+{$I SDL_mutex.inc}                        // 3.2.12