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)

.gitignore

@@ -88,6 +88,9 @@ logs/
 *.sln
 *.vcxproj*
 
+# Custom SCons configuration override
+/custom.py
+
 # Build results
 [Dd]ebug/
 [Dd]ebugPublic/