selftests/bpf: Fix compiling kfree_skb.c with musl-libc

The GNU version of 'struct tcphdr' with member 'doff' is not exposed by
musl headers unless _GNU_SOURCE is defined. Add this definition to fix
errors seen compiling for mips64el/musl-libc:

  In file included from kfree_skb.c:2:
  kfree_skb.c: In function 'on_sample':
  kfree_skb.c:45:30: error: 'struct tcphdr' has no member named 'doff'
     45 |         if (CHECK(pkt_v6->tcp.doff != 5, "check_tcp",
        |                              ^

Fixes: 580d656d ("selftests/bpf: Add kfree_skb raw_tp test")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/e2d8cedc790959c10d6822a51f01a7a3616bea1b.1721713597.git.tony.ambardar@gmail.com

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

 // SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
 #include <test_progs.h>
 #include <network_helpers.h>
 #include "kfree_skb.skel.h"