GTPv1U SerializeTo: GTPExtensionHeaders out of place

When encoding a packet that contains a GTPv1U layer that is not the last layer, the GTPExtensionHeaders are incorrectly placed at the end of the packet, instead of right after the GTPv1U minimum header before the next layer.

## Environment

Go v1.22.4
GoPacket v1.1.19

## Snippet to Reproduce

https://go.dev/play/p/E97DgsjS18d

```go
package main

import (
"encoding/hex"
"fmt"
"net"

"github.com/google/gopacket"
"github.com/google/gopacket/layers"
)

func main() {
payload := gopacket.Payload{0xA0, 0xA1, 0xA2, 0xA3}
udpI := &layers.UDP{SrcPort: 6363, DstPort: 6363}
ip4I := &layers.IPv4{Version: 4, TTL: 64, Protocol: layers.IPProtocolUDP, SrcIP: net.ParseIP("192.168.60.4"), DstIP: net.ParseIP("192.168.60.3")}
gtpExt := []layers.GTPExtensionHeader{{Type: 0x85, Content: []byte{10, 01}}}
gtp := &layers.GTPv1U{Version: 1, ProtocolType: 1, MessageType: 0xFF, MessageLength: uint16(8 + 20 + 8 + len(payload)), TEID: 0x10000008, GTPExtensionHeaders: gtpExt}
udp := &layers.UDP{SrcPort: 2152, DstPort: 2152}
ip4 := &layers.IPv4{Version: 4, TTL: 64, Protocol: layers.IPProtocolUDP, SrcIP: net.ParseIP("192.168.37.2"), DstIP: net.ParseIP("192.168.37.1")}
eth := &layers.Ethernet{SrcMAC: net.HardwareAddr{0x02, 0x00, 0x00, 0x00, 0x00, 0x02}, DstMAC: net.HardwareAddr{0x02, 0x00, 0x00, 0x00, 0x00, 0x01}, EthernetType: layers.EthernetTypeIPv4}
udp.SetNetworkLayerForChecksum(ip4)
udpI.SetNetworkLayerForChecksum(ip4I)

buf := gopacket.NewSerializeBuffer()
if e := gopacket.SerializeLayers(buf, gopacket.SerializeOptions{
FixLengths: true,
ComputeChecksums: true,
}, eth, ip4, udp, gtp, ip4I, udpI, payload); e != nil {
panic(e)
}

fmt.Println(hex.EncodeToString(buf.Bytes()))
}
```

## Expected Output

```text
0200000000010200000000020800 Ethernet
4500004c000000004011af4dc0a82502c0a82501 outer-IPv4
086808680038d511 outer-UDP
34ff00281000000800000085010a0100 GTPv1U
450000200000000040118175c0a83c04c0a83c03 inner-IPv4
18db18db000c9182 inner-UDP
a0a1a2a3 payload
```

## Actual Output

```text
0200000000010200000000020800 Ethernet
4500004c000000004011af4dc0a82502c0a82501 outer-IPv4
086808680038d511 outer-UDP
34ff002810000008 GTPv1U:header
450000200000000040118175c0a83c04c0a83c03 inner-IPv4
18db18db000c9182 inner-UDP
a0a1a2a3 payload
00000085010a0100 GTPv1U:extension
```

## Root Cause

https://github.com/google/gopacket/blob/32ee38206866f44a74a6033ec26aeeb474506804/layers/gtp.go#L132
`SerializeBuffer.AppendBytes` is called on a non-empty buffer, so that extension headers are placed after the inner layers.