GPencil: Make is_nofill_stroke property editable

This can be used by some add-ons.

source/blender/makesrna/intern/rna_gpencil.c

@@ -1187,7 +1187,6 @@ static void rna_def_gpencil_stroke(BlenderRNA *brna)
    * (this is a special flag for fill brush). */
   prop = RNA_def_property(srna, "is_nofill_stroke", PROP_BOOLEAN, PROP_NONE);
   RNA_def_property_boolean_sdna(prop, NULL, "flag", GP_STROKE_NOFILL);
-  RNA_def_property_clear_flag(prop, PROP_EDITABLE);
   RNA_def_property_ui_text(prop, "No Fill", "Special stroke to use as boundary for filling areas");
   RNA_def_property_update(prop, 0, "rna_GPencil_update");