debian: install linux-sysctl-defaults

This is only recommended, so mkosi does not pull it in, but
it sets the default sysctl from the kernel team so pull it in

Author: bluca

mkosi.conf.d/debian/mkosi.conf

@@ -28,6 +28,7 @@ Packages=
         libqrencode4
         linux-image-generic
         linux-perf
+        linux-sysctl-defaults
         login
         openssh-client
         openssh-server