systemtap: fix QA issue

systemtap: fix QA issue

ERROR: systemtap-4.0-r0 do_package: QA Issue: systemtap:
Files/directories were installed but not shipped in any package:
  /lib
  /lib/systemd
  /lib/systemd/system
  /lib/systemd/system/stap-exporter.service
Please set FILES such that these items are packaged. Alternatively if
they are unneeded, avoid installing them or delete them within
do_install.
systemtap: 4 installed and not shipped files. [installed-vs-shipped]

(From OE-Core rev: d31212bcfe9232e7f6ac9efb4fad0be175a9ca00)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>