Fix net assignment messed up in Eagle import

When there is a polygon in the Eagle project withput any pad connected
netCode was not incremented and so the wrong nets were assigned to the
following elements.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/21243

(cherry picked from commit 57ad779797)

pcbnew/pcb_io/eagle/pcb_io_eagle.cpp

@@ -2929,10 +2929,9 @@ void PCB_IO_EAGLE::loadSignals( wxXmlNode* aSignals )
 
             // therefore omit this signal/net.
         }
-        else
-        {
-            netCode++;
-        }
+
+        //Next signal needs a new netCode
+        netCode++;
 
         // Get next signal
         net = net->GetNext();