add include guard for SDRAM.h

Author: tokiedokie

libraries/Portenta_SDRAM/src/SDRAM.h

@@ -1,3 +1,6 @@
+#ifndef __SDRAM_H
+#define __SDRAM_H
+
 #include "ea_malloc.h"
 
 #ifdef __cplusplus
@@ -35,4 +38,5 @@ class SDRAMClass {
 
 extern SDRAMClass SDRAM;
 
-#endif
+#endif
+#endif // __SDRAM_H