Butter filter: needs header guards

libraries/Filter/Butter.h

@@ -1,3 +1,7 @@
+
+#ifndef __FILTER_BUTTER_H__
+#define __FILTER_BUTTER_H__
+
 template <typename Coefficients>
 class Butter2
 {
@@ -101,3 +105,5 @@ struct butter50_8_coeffs
 };
 typedef Butter2<butter50_8_coeffs> butter50hz8_0; //50hz sample, 8hz fcut
 typedef Butter2<butter50_8_coeffs> butter10hz1_6; //10hz sample, 1.6hz fcut
+
+#endif // __FILTER_BUTTER_H__