xfce4-notifyd: Ensure xfce4-notifyd subdirectory exists

The notify-dbus.h file is autogenerated during compilation into the
xfce4-notifyd subdirectory.  This directory doesn't exist in an
out-of-tree build so we add one manually.

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>