Pose Library: add category to `DOPESHEET_PT_asset_panel` panel

Add `bl_category` to the animation editor panel of the pose library. This
doesn't change anything in the GUI, but it does avoid the warning that
no category was set.

No functional changes.

pose_library/gui.py

@@ -167,6 +167,7 @@ class DOPESHEET_PT_asset_panel(Panel):
     bl_space_type = "DOPESHEET_EDITOR"
     bl_region_type = "UI"
     bl_label = "Create Pose Asset"
+    bl_category = "Pose Library"
 
     @classmethod
     def poll(cls, context: Context) -> bool: