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/pthread/SDL_sysrwlock.c

@@ -69,7 +69,7 @@ void SDL_LockRWLockForWriting(SDL_RWLock *rwlock) SDL_NO_THREAD_SAFETY_ANALYSIS
     }
 }
 
-SDL_bool SDL_TryLockRWLockForReading(SDL_RWLock *rwlock)
+bool SDL_TryLockRWLockForReading(SDL_RWLock *rwlock)
 {
     bool result = true;
 
@@ -86,7 +86,7 @@ SDL_bool SDL_TryLockRWLockForReading(SDL_RWLock *rwlock)
     return result;
 }
 
-SDL_bool SDL_TryLockRWLockForWriting(SDL_RWLock *rwlock)
+bool SDL_TryLockRWLockForWriting(SDL_RWLock *rwlock)
 {
     bool result = true;