- Add Conflicts with systemtap-sdt-devel, as that one again
provides the headers as well.

- Care for pecularities of build counter syncs between different
build rings: the Requires from sdt-devel can't use %release
for systemtap-headers, but we also don't want to just include
%version, so separate both totally:
* Add the include headers to sdt-devel again.
* Add Conflicts with systemtap-headers to care for the resulting
file conflicts.
* Remove Require from sdt-devel to systemtap-headers again.