Set the parent index of the root bone to 0 instead of -1

In normal UE2 derivates, it doesn't appear to matter what the parent index is; but in UE1 games that have basic support for PSK importing (i.e. Harry Potter 2), the parent index of the root bone must be `0` otherwise the import fails.

io_scene_psk_psa/psa/builder.py

@@ -98,7 +98,7 @@ def build_psa(context: bpy.types.Context, options: PsaBuildOptions) -> Psa:
             psa_bone.parent_index = parent_index
             psa.bones[parent_index].children_count += 1
         except ValueError:
-            psa_bone.parent_index = -1
+            psa_bone.parent_index = 0
 
         if bone.parent is not None:
             rotation = bone.matrix.to_quaternion().conjugated()