gsch2pcb: complains if it hits split symbols

[KaiMartin]

It is a recomended technique to split power pins of integrated circuits into their own symbol. If this symbol is written to *.sch before the original part, gsch2pcb complains if there is no footprint attribute. E.g.:
WARNING: 1U101 has no footprint attribute so won't be in the layout.

However, it is clearly better not to provide a redundant footprint attribute. gschem2pcb should check all of group of symbols with the same refdes. It should only complain if the footprints differ or if there is none at all.

---<(kaimartin)>---