appimage: Strip libfreetype & libz

As recommended in multiple tickets dealing with problems with those
libraries part of the AppImage.

Problems experienced on ArchLinux:
   /usr/lib/libfontconfig.so.1: error: symbol lookup error: undefined symbol: FT_Done_MM_Var (fatal)

... and after rm'ing just libfreetype:
   /tmp/.mount_KDevela4PEc7/usr/lib/libz.so.1: error: version lookup error: version `ZLIB_1.2.9' not found (required by /usr/lib/libpng16.so.16) (fatal)

Those libraries are also all blacklisted here:
  https://github.com/AppImage/AppImages/blob/master/excludelist#L81-L83