Add missing extern "C" to header files (closes #104)

src/rp2_common/hardware_rtc/include/hardware/rtc.h

@@ -28,6 +28,9 @@
  * \include hello_rtc.c
  */
 
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 typedef void (*rtc_callback_t)(void);
 
@@ -71,4 +74,8 @@ void rtc_set_alarm(datetime_t *t, rtc_callback_t user_callback);
  */
 void rtc_disable_alarm(void);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif