fix mockgen script which caused a probem with the vendored curve25519
fix mockgen script which caused a probem with the vendored curve25519

curve25519 uses Go compiler directives (go:noescape), which apparently
mockgen doesn't understand and therefore throws errors.

konnykonny50/quic-go,konnykonny50/quic-go,lucas-clemente/quic-go,lucas-clemente/quic-go