Fix ERC error missing on stacked pins

Stacked pins are not connected, so they should be flagged as such.  This
checks for stacked pins that are not part of different symbols

Fixes https://gitlab.com/kicad/code/kicad/issues/11926

(cherry picked from commit 90ef84b958)

eeschema/connection_graph.cpp

@@ -2666,7 +2666,8 @@ bool CONNECTION_GRAPH::ercCheckNoConnects( const CONNECTION_SUBGRAPH* aSubgraph
             {
             case SCH_PIN_T:
             {
-                if( !pins.empty() )
+                // Only consider a connection to be between pins on different symbols
+                if( !pins.empty() && ( item->GetParent() != pins.front()->GetParent() ) )
                     has_other_connections = true;
 
                 pins.emplace_back( static_cast<SCH_PIN*>( item ) );