The source refers to KWebPluginFactory::extractGuessedMimeType which is missing in the 4.8 release and as such fails to build against kdelibs-4.8.1-r2.
Thanks, fixed. Please sync. http://git.overlays.gentoo.org/gitweb/?p=proj/kde.git;a=commit;h=1d24e47ce348c71bba2164b0dd08b14eb433d40f