curl_ntlm_core: pass unsigned char to toupper

Otherwise, clang on Cygwin64 warns:
curl_ntlm_core.c:525:35: error: array subscript is of type 'char'
[-Werror,-Wchar-subscripts]
    dest[2 * i] = (unsigned char)(toupper(src[i]));
                                  ^~~~~~~~~~~~~~~
/usr/include/ctype.h:152:25: note: expanded from macro 'toupper'
      (void) __CTYPE_PTR[__x]; (toupper) (__x);})
                        ^~~~