Fix unicode filename compare when wchar_t is not 16bit

Do not use wcscmp() which takes wchar_t arguments.
wchar_t bit width is system dependent. It is often 32bits.
Grafx2 supports 16bits wide characters

fixes b49fa1dfcc

src/unicode.h

@@ -46,6 +46,9 @@ void Unicode_strlcpy(word * dst, const word * src, size_t len);
 /// Append unicode string to another
 void Unicode_strlcat(word * dst, const word * src, size_t len);
 
+/// Compare two unicode strings
+int Unicode_strcmp(const word * s1, const word * s2);
+
 /// Compare an unicode string with a regular Latin1 string
 int Unicode_char_strcmp(const word * s1, const char * s2);