e-wm: add setxkbmap to RDEPENDS

* called from xkb map E17 module
Jan 02 02:58:42 gjama xinit[418]: SET XKB RUN: setxkbmap  -model 'default'
Jan 02 02:58:42 gjama xinit[418]: SET XKB RUN: setxkbmap  -model 'default'
Jan 02 02:58:43 gjama xinit[418]: /bin/sh: setxkbmap: command not found
Jan 02 02:58:43 gjama xinit[418]: /bin/sh: setxkbmap: command not found

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