Removed SDL_bool in favor of plain bool

We require stdbool.h in the build environment, so we might as well use the plain bool type.

If your environment doesn't have stdbool.h, this simple replacement will suffice:
typedef signed char bool;

src/thread/generic/SDL_sysmutex.c

@@ -81,7 +81,7 @@ void SDL_LockMutex(SDL_Mutex *mutex) SDL_NO_THREAD_SAFETY_ANALYSIS  // clang doe
 #endif // SDL_THREADS_DISABLED
 }
 
-SDL_bool SDL_TryLockMutex(SDL_Mutex *mutex)
+bool SDL_TryLockMutex(SDL_Mutex *mutex)
 {
     bool result = true;
 #ifndef SDL_THREADS_DISABLED