Remove extant reference to the `symbols/extras' directory

Remove extant reference to the `symbols/extras' directory

The following command:

  setxkbmap -layout us -variant alt-intl-unicode

was yielding this unhelpful error:

  Error loading new keyboard description":

The reason is that the following commit:

  commit f836c210a4e4007715b149d4736a447c56b7cbaa
  Author: Sergey V. Udaltsov <svu@gnome.org>
  Date:   Wed Mar 2 23:56:25 2011 +0000

      The extra symbols are merged back into the base files

      The rules are not powerful enough to support the extras subdir.
      So the variants are merged into "main" files.
      From now on, the "extra" term only affects the directory: whether
      the record is in base.xml.in or base.extras.xml.in

removed the `symbols/extras' directory, but accidentally left a
reference to it in the definition of xkb_symbols "alt-intl-unicode":

  include "extras/us(intl-unicode)"