Fixed sdmmc host check to pass CI

Author: lbernstone

Diff for: libraries/SD_MMC/src/SD_MMC.h

@@ -17,7 +17,9 @@
 #include "sdkconfig.h"
 #include "soc/soc_caps.h"
 #ifndef SOC_SDMMC_HOST_SUPPORTED
-#error The SDMMC library requires a device with an SDIO Host
+#ifdef ARDUINO
+#warning The SDMMC library requires a device with an SDIO Host
+#endif
 #else
 
 #include "FS.h"