libxfce4ui: add RDEPENDS on PN for PN-gtk2 and PN-gtk3

gtk libs was separated into PN-gtk after commit:
d4e5b330

 libxfce4ui: build gtk3 libs and pack gtk2/gtk3 libs in separate packages

but config files such as xfce4-keyboard-shortcuts.xml
is in PN, it works without this, but there no any keyboard
shortcut, add the RDEPENDS so that the libs and config
files will be both installed.

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Acked-By: Andreas Müller <schnitzeltony@goolglemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>