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.

contrib/lax_der_privatekey_parsing.h

@@ -25,14 +25,14 @@
  * library are sufficient.
  */
 
-#ifndef _SECP256K1_CONTRIB_BER_PRIVATEKEY_H_
-#define _SECP256K1_CONTRIB_BER_PRIVATEKEY_H_
+#ifndef SECP256K1_CONTRIB_BER_PRIVATEKEY_H
+#define SECP256K1_CONTRIB_BER_PRIVATEKEY_H
 
 #include <secp256k1.h>
 
-# ifdef __cplusplus
+#ifdef __cplusplus
 extern "C" {
-# endif
+#endif
 
 /** Export a private key in DER format.
  *
@@ -87,4 +87,4 @@ SECP256K1_WARN_UNUSED_RESULT int ec_privkey_import_der(
 }
 #endif
 
-#endif
+#endif /* SECP256K1_CONTRIB_BER_PRIVATEKEY_H */