Use SfxStyleSheetIterator to iterate over SfxStyles in sd/pptin.cxx

Change-Id: I99534efdd52644882f2e2960bdafce58bf632804
Reviewed-on: https://gerrit.libreoffice.org/8483
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>