crypto/secp256k1: fix 32-bit tests when CGO_ENABLED=0 (#28602)

Author: gballet

crypto/secp256k1/secp256_test.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be found in
 // the LICENSE file.
 
+//go:build !gofuzz && cgo
+// +build !gofuzz,cgo
+
 package secp256k1
 
 import (