rxvt-unicode: fix for 9.20 and 9.21

rxvt-unicode: fix for 9.20 and 9.21

* Add a rxvt-unicode.inc for rxvt-unicode_9.20.bb and rxvt-unicode_9.21.bb
* Fix unknown-configure-option:
  WARNING: QA Issue: rxvt-unicode: configure was passed unrecognised options: --enable-menubar --enable-cursor-blink --enable-shared --disable-strings --enable-xgetdefault [unknown-configure-option]
* Fix build-deps: (add a PACKAGECONFIG)
  WARNING: QA Issue: rxvt-unicode rdepends on startup-notification, but it isn't a build dependency? [build-deps]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>