Remove underscores from header defs.

This makes them consistent with other files and avoids reserved identifiers.

src/scratch.h

@@ -4,8 +4,8 @@
  * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
  ***********************************************************************/
 
-#ifndef _SECP256K1_SCRATCH_
-#define _SECP256K1_SCRATCH_
+#ifndef SECP256K1_SCRATCH_H
+#define SECP256K1_SCRATCH_H
 
 /* The typedef is used internally; the struct name is used in the public API
  * (where it is exposed as a different typedef) */