sgml: address signed/unsigned warnings

The sgml_unicode.hpp stores UTF-32 strings in
stdex::sgml_unicode_pair::unicode.  Hence stdex::utf32_t should be used as
its datatype.  Unfortunately, that doesn't work with U string literal: U
requires hundreds of typecasts to make it fit stdex::utf32_t, since char32_t
is signed, stdex::utf32_t is unsigned.  OTOH, L string literal cannot be
used on Windows (produces UTF-16, not UTF-32).

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

include/stdex/sgml.hpp

@@ -41,7 +41,7 @@ namespace stdex
 						j = m;
 					else {
 						for (; i < m && strncmp<char, T>(sgml_unicode[m - 1].sgml, _countof(sgml_unicode[0].sgml), entity, count) == 0; m--);
-						return reinterpret_cast<const utf32_t*>(sgml_unicode[m].unicode);
+						return sgml_unicode[m].unicode;
 					}
 				}
 			}