Add warning to make non-registered channelOpenCb visible #311

Job	Run time
Build OpenSSH	3s
Build wolfSSL (v5.8.4-stable)	4s
Build wolfSSL (master)	6s
Build and test wolfSSH (v5.8.4-stable, --enable-smallstack)	25s
Build and test wolfSSH (v5.8.4-stable)	26s
Build and test wolfSSH (master)	27s
Build and test wolfSSH (master, --enable-smallstack)	23s
	1m 54s

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