SOGoUI

With GCC 10, SOGoAptFormatter.m FTBS with the following error:

    SOGoAptFormatter.m: In function ‘-[SOGoAptFormatter setPrivateTitleOnly]’:
    SOGoAptFormatter.m:104:56: error: expected ‘)’ before ‘::’ token
      104 |   self->formatAction = @selector(titleOnlyForPrivateApt::);

Simply name the second argument, so gcc is happy.

Also add the `external` keyword to a header declared variable.

Fixes #5029