Add warning to make non-registered channelOpenCb visible #270

Job	Run time
build (basic, wolfssh-windows-build)	1m 41s
build (large_rw, wolfssh-windows-build-large-rw)	1m 31s
test (basic, wolfssh-windows-build)	47s
test (large_rw, wolfssh-windows-build-large-rw)	7m 54s
	11m 53s

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