mkosi: Add a postinstall script to setup .gdbinit.

systemd is compiled with relative source file paths so we need to
issue a directory command to make sure gdb can find systemd's
source files. Let's put this in a .gdbinit file so it's executed
automatically when we run gdb.

mkosi.postinst

@@ -0,0 +1,5 @@
+#!/bin/sh
+set -e
+
+mkdir -p /root/build
+echo "directory /root/build" > ~/.gdbinit