libcopybit: do not use GNU old-style field designators

Summary: Not using a GNU extension improves portability

Change-Id: Iae511818ee2d6b9a0391327a19136836fc29fc3d
libcopybit: do not use GNU old-style field designators

Change-Id: I379ecb584b1d5baa377dd1438c0ae97ae84d9aa0