Ruby

Specify int bitfield as signed int bitfield

sunc treats int bitfield as unsigned int.
This commit will fix build failure on sunc.

• http://rubyci.s3.amazonaws.com/solaris10-sunc/ruby-master/log/20230617T100003Z.fail.html.gz
• http://rubyci.s3.amazonaws.com/solaris11-sunc/ruby-master/log/20230617T090011Z.fail.html.gz