builddeb: remove unneeded files in hdrobjfiles for headers package

- We do not need tools/objtool/fixdep or tools/objtool/sync-check.sh
   for building external modules. Including tools/objtool/objtool is
   enough.

 - gcc-common.h is a check-in file. I do not see any point to search
   for it in objtree.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>