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/ecmult_impl.h

@@ -4,8 +4,8 @@
  * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
  **********************************************************************/
 
-#ifndef _SECP256K1_ECMULT_IMPL_H_
-#define _SECP256K1_ECMULT_IMPL_H_
+#ifndef SECP256K1_ECMULT_IMPL_H
+#define SECP256K1_ECMULT_IMPL_H
 
 #include <string.h>
 
@@ -403,4 +403,4 @@ static void secp256k1_ecmult(const secp256k1_ecmult_context *ctx, secp256k1_gej
     }
 }
 
-#endif
+#endif /* SECP256K1_ECMULT_IMPL_H */