mighttpd2.cabal: rename mkindex to 'mighty-mkindex'

To avoid systemwide installation collision.

mkindex is tool name of commonly used latex package:
[sf] ~:equery b mkindex
> * Searching for mkindex ...
>app-text/texlive-core-2012-r1 (/usr/bin/mkindex)

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>