Add `custom.py` to .gitignore
Add `custom.py` to .gitignore

The default `custom.py` can be created at the root of the Godot
repository to initialize any SCons build options via file which are
specific to user.

(cherry picked from commit a712e14038736500c780c3090558ee4185b80219)

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot