Don't accept any values not in the dropdown for alternate pins.

Fixes https://gitlab.com/kicad/code/kicad/issues/5399

eeschema/dialogs/dialog_sch_pin_table.cpp

@@ -74,7 +74,7 @@ public:
                 choices.push_back( alt.first );
 
             wxGridCellAttr* attr = new wxGridCellAttr();
-            attr->SetEditor( new GRID_CELL_COMBOBOX( choices ) );
+            attr->SetEditor( new wxGridCellChoiceEditor( choices ) );
 
             m_nameAttrs.push_back( attr );
         }