Fix header guards using reserved identifiers

Identifiers starting with an underscore and followed immediately by a capital letter are reserved by the C++ standard.

The only header guards not fixed are those in the headers auto-generated from java.

src/util.h

@@ -4,8 +4,8 @@
  * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
  **********************************************************************/
 
-#ifndef _SECP256K1_UTIL_H_
-#define _SECP256K1_UTIL_H_
+#ifndef SECP256K1_UTIL_H
+#define SECP256K1_UTIL_H
 
 #if defined HAVE_CONFIG_H
 #include "libsecp256k1-config.h"
@@ -110,4 +110,4 @@ static SECP256K1_INLINE void *checked_malloc(const secp256k1_callback* cb, size_
 SECP256K1_GNUC_EXT typedef unsigned __int128 uint128_t;
 #endif
 
-#endif
+#endif /* SECP256K1_UTIL_H */