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/scalar_low.h

@@ -4,12 +4,12 @@
  * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
  **********************************************************************/
 
-#ifndef _SECP256K1_SCALAR_REPR_
-#define _SECP256K1_SCALAR_REPR_
+#ifndef SECP256K1_SCALAR_REPR_H
+#define SECP256K1_SCALAR_REPR_H
 
 #include <stdint.h>
 
 /** A scalar modulo the group order of the secp256k1 curve. */
 typedef uint32_t secp256k1_scalar;
 
-#endif
+#endif /* SECP256K1_SCALAR_REPR_H */