added ifdef-guards

Author: dderjoel

src/third_party/secp256k1_dettman_64.h

@@ -11,6 +11,9 @@
 /*  */
 /*  */
 
+#ifndef SECP256K1_DETTMAN_64_H
+#define SECP256K1_DETTMAN_64_H
+
 #include <stdint.h>
 #if defined(__GNUC__) || defined(__clang__)
 #  define FIAT_SECP256K1_DETTMAN_FIAT_EXTENSION __extension__
@@ -199,3 +202,5 @@ static FIAT_SECP256K1_DETTMAN_FIAT_INLINE void fiat_secp256k1_dettman_square(uin
   out1[3] = x34;
   out1[4] = x35;
 }
+
+#endif