gnome-extra/xmonad-log-applet: use github mirror as suggested by repoman

  SRC_URI.mirror                1
   gnome-extra/xmonad-log-applet/xmonad-log-applet-2.0.0.ebuild: 'https://github.com/downloads/' found in thirdpartymirrors, use 'mirror://github/alexkay/xmonad-log-applet/xmonad-log-applet-2.0.0.tar.gz'

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>