bpf, tls: add tls header to tools infrastructure

Andrey reported a build error for the BPF kselftest suite when compiled on
a machine which does not have tls related header bits installed natively:

  test_sockmap.c:120:23: fatal error: linux/tls.h: No such file or directory
   #include <linux/tls.h>
                         ^
  compilation terminated.

Fix it by adding the header to the tools include infrastructure and add
definitions such as SOL_TLS that could potentially be missing.

Fixes: e9dd9047

 ("bpf: add tls support for testing in test_sockmap")
Reported-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>