add header guards to buffer h

buffer.h

@@ -1,3 +1,6 @@
+#ifndef _BUFFER_H_
+#define _BUFFER_H_
+
 #include <stdint.h>
 
 #define BUFFER_SIZE 200
@@ -9,3 +12,5 @@ typedef struct {
 
 void insert_to_buffer(uint16_t val, volatile buffer_t* buf);
 float get_buffer_mean(volatile buffer_t* buf);
+
+#endif//_BUFFER_H_