mmc: sdhci-s3c: hide forward declaration of of_device_id behind CONFIG_OF

The struct of_device_id is not defined with !CONFIG_OF so its forward
declaration should be hidden to as well.  This should address clang
compile warning:

  drivers/mmc/host/sdhci-s3c.c:464:34: warning: tentative array definition assumed to have one element

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200925072532.10272-1-krzk@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/sdhci-s3c.c

@@ -461,7 +461,9 @@ static int sdhci_s3c_parse_dt(struct device *dev,
 }
 #endif
 
+#ifdef CONFIG_OF
 static const struct of_device_id sdhci_s3c_dt_match[];
+#endif
 
 static inline struct sdhci_s3c_drv_data *sdhci_s3c_get_driver_data(
 			struct platform_device *pdev)