particles: Fix regression loading .ptf files

Regression introduced in #1454

direct/src/particles/ParticleEffect.py

@@ -2,6 +2,7 @@
 from panda3d.core import Filename, NodePath, VirtualFileSystem, getModelPath
 
 # Leave these imports in, they may be used by ptf files.
+from panda3d.core import * # pylint: disable=unused-import
 from panda3d.physics import * # pylint: disable=unused-import
 from . import Particles # pylint: disable=unused-import
 from . import ForceGroup # pylint: disable=unused-import