dt-bindings: ili9881c: add missing panel-common inheritance

The properties below refer to items in panel-common.yaml, which means
that needs to be referenced in the schema.
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020153432.383845-2-john@metanate.com

Documentation/devicetree/bindings/display/panel/ilitek,ili9881c.yaml

@@ -9,6 +9,9 @@ title: Ilitek ILI9881c based MIPI-DSI panels
 maintainers:
   - Maxime Ripard <mripard@kernel.org>
 
+allOf:
+  - $ref: panel-common.yaml#
+
 properties:
   compatible:
     items: