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: cfa7b011894d ("selftests/bpf: tests for using dynptrs to parse skb and xdp buffers")
Signed-off-by: Tony Ambardar <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/ac544[e-mail]

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"