fix(rigging): weight painting with external parent bone

[GixoXYZ]

System Information
Operating system: All

Blender Version
Broken: All
Worked: -

GBH Tool Version
Broken: 2.2.0.alpha.3
Worked: -

Short description of error
When using automatic weight painting, the external parent bone won't work.

Exact steps for others to reproduce the error

1. Create a mesh hair with its armature.
2. Set parent bone to "Select Parent Bone from Another Armature" and select the bone.
3. Click on "Automatic Weight Paint".