presets: enable systemd-pstore.service by default

It has no effect is the pstore is not used, and prevents the non-volatile
storage from filling up if is used by the kernel.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=952767
(cherry picked from commit 5926ea0)

Resolves: #2217786

presets/90-systemd.preset

@@ -34,3 +34,5 @@ disable syslog.socket
 disable systemd-journal-gatewayd.*
 disable systemd-journal-remote.*
 disable systemd-journal-upload.*
+
+enable systemd-pstore.service