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/num_gmp_impl.h

@@ -4,8 +4,8 @@
  * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
  **********************************************************************/
 
-#ifndef _SECP256K1_NUM_REPR_IMPL_H_
-#define _SECP256K1_NUM_REPR_IMPL_H_
+#ifndef SECP256K1_NUM_REPR_IMPL_H
+#define SECP256K1_NUM_REPR_IMPL_H
 
 #include <string.h>
 #include <stdlib.h>
@@ -285,4 +285,4 @@ static void secp256k1_num_negate(secp256k1_num *r) {
     r->neg ^= 1;
 }
 
-#endif
+#endif /* SECP256K1_NUM_REPR_IMPL_H */