regulator: stm32-vrefbuf: Fix W=1 build warning when CONFIG_OF=n

Fix below warning when CONFIG_OF=n:

drivers/regulator/stm32-vrefbuf.c:287:34: warning: ‘stm32_vrefbuf_of_match’ defined but not used [-Wunused-const-variable=]
  287 | static const struct of_device_id stm32_vrefbuf_of_match[] = {
      |                                  ^~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Acked-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Link: https://lore.kernel.org/r/20200821111755.4c461039@xhacker.debian
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/stm32-vrefbuf.c

@@ -284,7 +284,7 @@ static const struct dev_pm_ops stm32_vrefbuf_pm_ops = {
 			   NULL)
 };
 
-static const struct of_device_id stm32_vrefbuf_of_match[] = {
+static const struct of_device_id __maybe_unused stm32_vrefbuf_of_match[] = {
 	{ .compatible = "st,stm32-vrefbuf", },
 	{},
 };