sgml: Silence unreferenced parameter warning

This parameter is needed to provide compatibility between utf16_t and
utf32_t, so we may call this function from a single template for both
types.

Signed-off-by: Simon Rozman <simon@rozman.si>

include/stdex/sgml.hpp

@@ -532,6 +532,7 @@ namespace stdex
 
 	inline utf32_t wstr_to_utf32(_In_reads_(end) const utf32_t* src, _Inout_ size_t& i, _In_ size_t end)
 	{
+		_Unreferenced_(end);
 		_Assume_(i < end);
 		return src[i++];
 	}