Allow usage of the new Condition Variable code with Critical Sections

Vista and later provide the SleepConditionVariableCS() function for this.

Since SDL_syscond_srw.c doesn't require SRW locks anymore, rename it to
SDL_syscond_cv.c which better reflects the implementation of condition
variables rather than the implementation of mutexes.

Fixes #4051.

src/thread/windows/SDL_sysmutex.c

@@ -169,11 +169,6 @@ static const SDL_mutex_impl_t SDL_mutex_impl_srw =
  * Fallback Mutex implementation using Critical Sections (before Win 7)
  */
 
-typedef struct SDL_mutex_cs
-{
-    CRITICAL_SECTION cs;
-} SDL_mutex_cs;
-
 /* Create a mutex */
 static SDL_mutex *
 SDL_CreateMutex_cs(void)