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/testrand.h

@@ -4,8 +4,8 @@
  * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
  **********************************************************************/
 
-#ifndef _SECP256K1_TESTRAND_H_
-#define _SECP256K1_TESTRAND_H_
+#ifndef SECP256K1_TESTRAND_H
+#define SECP256K1_TESTRAND_H
 
 #if defined HAVE_CONFIG_H
 #include "libsecp256k1-config.h"
@@ -35,4 +35,4 @@ static void secp256k1_rand256_test(unsigned char *b32);
 /** Generate pseudorandom bytes with long sequences of zero and one bits. */
 static void secp256k1_rand_bytes_test(unsigned char *bytes, size_t len);
 
-#endif
+#endif /* SECP256K1_TESTRAND_H */