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.

.gitignore

@@ -89,6 +89,9 @@ logs/
 *.sln
 *.vcxproj*
 
+# Custom SCons configuration override
+/custom.py
+
 # Build results
 [Dd]ebug/
 [Dd]ebugPublic/