xinit: add rxvt-unicode in RDEPENDS

xinit: add rxvt-unicode in RDEPENDS

fix below error:
/etc/X11/xinit/xinitrc: line 55: exec: xterm: not found

by default, xinit will generate /etc/X11/xinit/xinitrc,
and have below config in it:
XTERM -geometry 80x50+494+51 &
XTERM -geometry 80x20+494-0 &
exec XTERM -geometry 80x66+0+0 -name login

the default XTERM is xterm, but xterm is in meta-oe,
use rxvt instead

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>