Fix lsl_get_channel_bytes to return 0 for cft_string as documented

[myd7349]

The documentation for lsl_get_channel_bytes states that it returns 0 for the string type (cft_string), but in practice this is not the case:

liblsl/include/lsl/streaminfo.h

Lines 174 to 175 in cdf917d

	/// Number of bytes occupied by a channel (0 for string-typed channels).
	extern LIBLSL_C_API int32_t lsl_get_channel_bytes(lsl_streaminfo info);

This patch fixes the issue by returning 0 for the string type.

[myd7349]

The endianness unit test appears to have failed. I will try to fix this issue.