fix(bom): use GetUniStringLibId() instead of str() for footprint in BOM export

str(module.GetFPID()) returns the Swig proxy representation like
"<pcbnew.LIB_ID; proxy of ...>" instead of the actual footprint name.
GetUniStringLibId() returns the proper "Library:Footprint" string.

python/commands/export.py

@@ -474,7 +474,7 @@ class ExportCommands:
                 component = {
                     "reference": module.GetReference(),
                     "value": module.GetValue(),
-                    "footprint": str(module.GetFPID()),
+                    "footprint": module.GetFPID().GetUniStringLibId(),
                     "layer": self.board.GetLayerName(module.GetLayer())
                 }