debian: build libc headers package only for linux-librem5
this change is for all branches, current and next.
this is an attempt to fix https://source.puri.sm/Librem5/linux-next/-/issues/186
the "next"-branch needs one additional patch that removes building the libc headers package from Linux: https://source.puri.sm/Librem5/linux-next/-/merge_requests/142
Edited by Martin Kepplinger