check-symfile: Use pythonesque string formatting instead of perl

Commit d30a1ad0443 translated the symbol file checker from perl to
python by doing a literal translation in most cases. Unfortunately one
string formatting operation was not really translated into python
leaving users with non-helpful error:

'Symbol $1 is listed twice'

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>