package/rsyslog: install default service file

package/rsyslog: install default service file

As of v8.2008 rsyslog no longer provides a default service file, and now
suggests using the platform suggested defaults. For Buildroot, install
the Debian service file which has been added in the same version,
however is not included in the official release.

Upstream commit which adds this service file:
https://github.com/rsyslog/rsyslog/commit/cfd07503ba055100a84d75d1a78a5c6cceb9fdab

Signed-off-by: Sam Voss <sam.voss@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>