Made ob param of `modifiers_getVirtualModifierList` const

The `modifiers_getVirtualModifierList()` function previously took a non-
const `Object *ob` parameter, preventing it from being called from more
restrictive functions. Since the function doesn't modify the passed
object, it could easily be made const.

No functional changes.

source/blender/blenkernel/BKE_modifier.h

@@ -417,7 +417,7 @@ typedef struct VirtualModifierData {
   ShapeKeyModifierData smd;
 } VirtualModifierData;
 
-struct ModifierData *modifiers_getVirtualModifierList(struct Object *ob,
+struct ModifierData *modifiers_getVirtualModifierList(const struct Object *ob,
                                                       struct VirtualModifierData *data);
 
 /* ensure modifier correctness when changing ob->data */

source/blender/blenkernel/intern/modifier.c

@@ -614,7 +614,7 @@ ModifierData *modifiers_getLastPreview(struct Scene *scene, ModifierData *md, in
 
 /* This is to include things that are not modifiers in the evaluation of the modifier stack, for
  * example parenting to an armature. */
-ModifierData *modifiers_getVirtualModifierList(Object *ob,
+ModifierData *modifiers_getVirtualModifierList(const Object *ob,
                                                VirtualModifierData *virtualModifierData)
 {
   ModifierData *md;