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
ericmckean/seccompsandbox,cnbird1999/seccompsandbox,ericmckean/seccompsandbox,cnbird1999/seccompsandbox,ericmckean/seccompsandbox,cnbird1999/seccompsandbox