selftests/bpf: Fix redefinition errors compiling lwt_reroute.c

Compiling lwt_reroute.c with GCC 12.3 for mips64el/musl-libc yields errors:

In file included from .../include/arpa/inet.h:9,
                 from ./test_progs.h:18,
                 from tools/testing/selftests/bpf/prog_tests/lwt_helpers.h:11,
                 from tools/testing/selftests/bpf/prog_tests/lwt_reroute.c:52:
.../include/netinet/in.h:23:8: error: redefinition of 'struct in6_addr'
   23 | struct in6_addr {
      |        ^~~~~~~~
In file included from .../include/linux/icmp.h:24,
                 from tools/testing/selftests/bpf/prog_tests/lwt_helpers.h:9:
.../include/linux/in6.h:33:8: note: originally defined here
   33 | struct in6_addr {
      |        ^~~~~~~~
.../include/netinet/in.h:34:8: error: redefinition of 'struct sockaddr_in6'
   34 | struct sockaddr_in6 {
      |        ^~~~~~~~~~~~
.../include/linux/in6.h:50:8: note: originally defined here
   50 | struct sockaddr_in6 {
      |        ^~~~~~~~~~~~
.../include/netinet/in.h:42:8: error: redefinition of 'struct ipv6_mreq'
   42 | struct ipv6_mreq {
      |        ^~~~~~~~~
.../include/linux/in6.h:60:8: note: originally defined here
   60 | struct ipv6_mreq {
      |        ^~~~~~~~~

These errors occur because <linux/in6.h> is included before <netinet/in.h>,
bypassing the Linux uapi/libc compat mechanism's partial musl support. As
described in [1] and [2], fix these errors by including <netinet/in.h> in
lwt_reroute.c before any uapi headers.

[1]: commit c0bace79 ("uapi libc compat: add fallback for unsupported libcs")
[2]: https://git.musl-libc.org/cgit/musl/commit/?id=04983f227238

Fixes: 6c77997b ("selftests/bpf: Add lwt_xmit tests for BPF_REROUTE")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/bd2908aec0755ba8b75f5dc41848b00585f5c73e.1722244708.git.tony.ambardar@gmail.com

tools/testing/selftests/bpf/prog_tests/lwt_reroute.c

@@ -49,6 +49,7 @@
  *  is not crashed, it is considered successful.
  */
 #define NETNS "ns_lwt_reroute"
+#include <netinet/in.h>
 #include "lwt_helpers.h"
 #include "network_helpers.h"
 #include <linux/net_tstamp.h>