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.

include/secp256k1.h

@@ -1,9 +1,9 @@
-#ifndef _SECP256K1_
-# define _SECP256K1_
+#ifndef SECP256K1_H
+#define SECP256K1_H
 
-# ifdef __cplusplus
+#ifdef __cplusplus
 extern "C" {
-# endif
+#endif
 
 #include <stddef.h>
 
@@ -614,8 +614,8 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_combine(
     size_t n
 ) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
 
-# ifdef __cplusplus
+#ifdef __cplusplus
 }
-# endif
-
 #endif
+
+#endif /* SECP256K1_H */