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

The GNU version of 'struct tcphdr', with members 'doff' and 'urg_ptr', is
not exposed by musl headers unless _GNU_SOURCE is defined.

Add this definition to fix errors seen compiling for mips64el/musl-libc:

  parse_tcp_hdr_opt.c:18:21: error: 'struct tcphdr' has no member named 'urg_ptr'
     18 |         .pk6_v6.tcp.urg_ptr = 123,
        |                     ^~~~~~~
  parse_tcp_hdr_opt.c:19:21: error: 'struct tcphdr' has no member named 'doff'
     19 |         .pk6_v6.tcp.doff = 9, /* 16 bytes of options */
        |                     ^~~~

Fixes: cfa7b011 ("selftests/bpf: tests for using dynptrs to parse skb and xdp buffers")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/ac5440213c242c62cb4e0d9e0a9cd5058b6a31f6.1721713597.git.tony.ambardar@gmail.com

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

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