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

The GNU version of 'struct tcphdr' has members 'doff', 'source' and 'dest',
which are not exposed by musl libc headers unless _GNU_SOURCE is defined.

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

  flow_dissector.c:118:30: error: 'struct tcphdr' has no member named 'doff'
    118 |                         .tcp.doff = 5,
        |                              ^~~~
  flow_dissector.c:119:30: error: 'struct tcphdr' has no member named 'source'
    119 |                         .tcp.source = 80,
        |                              ^~~~~~
  flow_dissector.c:120:30: error: 'struct tcphdr' has no member named 'dest'
    120 |                         .tcp.dest = 8080,
        |                              ^~~~

Fixes: ae173a91 ("selftests/bpf: support BPF_FLOW_DISSECTOR_F_PARSE_1ST_FRAG")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/8f7ab21a73f678f9cebd32b26c444a686e57414d.1721713597.git.tony.ambardar@gmail.com

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

 // SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
 #include <test_progs.h>
 #include <network_helpers.h>
 #include <linux/if_tun.h>