cpu/sam21_common: add C++ wrapper to sam0.h

cpu/sam21_common/include/sam0.h

@@ -19,11 +19,19 @@
 #ifndef SAM0_H
 #define SAM0_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #if defined(__SAMR21G18A__) || defined(__ATSAMR21G18A__)
 #include "cmsis/samr21/include/samr21g18a.h"
 #else
   #error "Unsupported SAM0 variant."
 #endif
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* SAM0_H */
 /** @} */