xfce4-power-manager: make upower a build dependency

This one came up when upower failed:

| *** The required package upower-glib was not found on your system.
| *** Please install upower-glib (atleast version 0.9.7) or adjust
| *** the PKG_CONFIG_PATH environment variable if you
| *** installed the package in a nonstandard prefix so that
| *** pkg-config is able to find it.

xfce4-power-manager links against upower so we can expect shlibs causing upower
being added to RDEPENDS.

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>