Fix T87486: Can't open file with modifiers to a grease pencil library override

Very stupid typo in override apply code on GP modifiers (typical
copy/paste mistake from original modifiers code).

@jbakker this should be back-ported to 2.93LTS.

source/blender/makesrna/intern/rna_object.c

@@ -1882,7 +1882,7 @@ bool rna_Object_greasepencil_modifiers_override_apply(Main *bmain,
   GpencilModifierData *mod_dst = ED_object_gpencil_modifier_add(
       NULL, bmain, NULL, ob_dst, mod_src->name, mod_src->type);
 
-  BLI_remlink(&ob_dst->modifiers, mod_dst);
+  BLI_remlink(&ob_dst->greasepencil_modifiers, mod_dst);
   /* This handles NULL anchor as expected by adding at head of list. */
   BLI_insertlinkafter(&ob_dst->greasepencil_modifiers, mod_anchor, mod_dst);