GH_BUILD_TYPE: no @DESCRIPTION found

[kwhat]

This overlay is causing build errors against app-portage/eclass-manpages-20150814-r1::gentoo due to github.eclass.

 * Generating man pages from: /var/lib/layman/mrueg/eclass

github.eclass:
   warning:34: GH_BUILD_TYPE: unable to extract default variable content: # @DEFAULT-UNSET
   error:34: GH_BUILD_TYPE: no @DESCRIPTION found

[mrueg]

There was a typo in @DEFAULT_UNSET, please test again.

[kwhat]

👍 Thanks!