boards/nrf51dongle: fixed include gurads

boards/nrf51dongle/include/periph_conf.h

@@ -16,8 +16,8 @@
  * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
  */
 
-#ifndef __PERIPH_CONF_H
-#define __PERIPH_CONF_H
+#ifndef PERIPH_CONF_H
+#define PERIPH_CONF_H
 
 #ifdef __cplusplus
 extern "C" {
@@ -120,4 +120,5 @@ extern "C" {
 }
 #endif
 
-#endif /* __PERIPH_CONF_H */
+#endif /* PERIPH_CONF_H */
+/** @} */