thread: Define ff_mutex_* macros as stub functions when threads are disabled

Silences a bunch of "statement with no effect" warnings with threads disabled.

libavutil/thread.h

@@ -48,10 +48,10 @@
 
 #define AVMutex char
 
-#define ff_mutex_init(mutex, attr) (0)
+static inline int ff_mutex_init(AVMutex *mutex, const void *attr){ return 0; }
-#define ff_mutex_lock(mutex) (0)
+static inline int ff_mutex_lock(AVMutex *mutex){ return 0; }
-#define ff_mutex_unlock(mutex) (0)
+static inline int ff_mutex_unlock(AVMutex *mutex){ return 0; }
-#define ff_mutex_destroy(mutex) (0)
+static inline int ff_mutex_destroy(AVMutex *mutex){ return 0; }
 
 #define AVOnce char
 #define AV_ONCE_INIT 0