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

The GNU version of 'struct tcp_info' in 'netinet/tcp.h' is not exposed by
musl headers unless _GNU_SOURCE is defined.

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

  tcp_rtt.c: In function 'wait_for_ack':
  tcp_rtt.c:24:25: error: storage size of 'info' isn't known
     24 |         struct tcp_info info;
        |                         ^~~~
  tcp_rtt.c:24:25: error: unused variable 'info' [-Werror=unused-variable]
  cc1: all warnings being treated as errors

Fixes: 1f4f80fe ("selftests/bpf: test_progs: convert test_tcp_rtt")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/f2329767b15df206f08a5776d35a47c37da855ae.1721713597.git.tony.ambardar@gmail.com

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

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