eliminate compiler warning when DEBUG_CHANNEL_LOCKS is enabled and users of this header file don't also include channel.h

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@43952 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/lock.h

@@ -705,6 +705,8 @@ AST_INLINE_API(int ast_atomic_dec_and_test(volatile int *p),
 #define ast_channel_trylock(x)		ast_mutex_trylock(&x->lock)
 #else
 
+struct ast_channel;
+
 /*! \brief Lock AST channel (and print debugging output)
 \note You need to enable DEBUG_CHANNEL_LOCKS for this function */
 int ast_channel_lock(struct ast_channel *chan);