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

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: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>