fix: crypto/keys/secp246k1/*: add +build cgo guard to avoid failure with CGO_ENABLED=0 (#13267)

Fixes an oversight that hadn't been considered when the build
guards were added to the other files in the same directory, that
this test requires linking with code that hooks into cgo. This change
adds the build guard

    +build cgo

Fixes #13266

crypto/keys/secp256k1/internal/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 (