notes.h: declare bit field as unsigned to silence compiler complaints

The IRIX MIPSPro compiler complains like this:

   cc-1107 c99: WARNING File = notes.h, Line = 215
     A signed bit field has a length of 1 bit.

           int suppress_default_notes:1;
               ^

'unsigned' is what was intended, so lets make it so.
Signed-off-by: NBrandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

notes.h

@@ -212,7 +212,7 @@ void format_note(struct notes_tree *t, const unsigned char *object_sha1,
 struct string_list;
 
 struct display_notes_opt {
-	int suppress_default_notes:1;
+	unsigned int suppress_default_notes:1;
 	struct string_list *extra_notes_refs;
 };