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/bench.h

@@ -4,8 +4,8 @@
  * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
  **********************************************************************/
 
-#ifndef _SECP256K1_BENCH_H_
-#define _SECP256K1_BENCH_H_
+#ifndef SECP256K1_BENCH_H
+#define SECP256K1_BENCH_H
 
 #include <stdio.h>
 #include <math.h>
@@ -63,4 +63,4 @@ void run_benchmark(char *name, void (*benchmark)(void*), void (*setup)(void*), v
     printf("us\n");
 }
 
-#endif
+#endif /* SECP256K1_BENCH_H */