Add DEPFLAGS to makefile in places missed before
Add DEPFLAGS to makefile in places missed before

This means changes to sandbox_impl.h will correctly cause
syscall_table.c to be rebuilt.

Also remove HEADERS references in places where a merge re-introduced them.

Review URL: http://codereview.chromium.org/3290010

git-svn-id: c2a6f690695ec6703fa4e0d44c2dfcc6697c486d@101 55e79e8e-603c-11de-8c10-5fe6993ea61f

mseaborn/seccompsandbox,mseaborn/seccompsandbox,mseaborn/seccompsandbox