selftests/bpf: Fix build of sockmap_ktls.c

The selftests fails to build with:
tools/testing/selftests/bpf/prog_tests/sockmap_ktls.c: In function ‘test_sockmap_ktls_disconnect_after_delete’:
tools/testing/selftests/bpf/prog_tests/sockmap_ktls.c:72:37: error: ‘TCP_ULP’ undeclared (first use in this function)
   72 |  err = setsockopt(cli, IPPROTO_TCP, TCP_ULP, "tls", strlen("tls"));
      |                                     ^~~~~~~

Similar to commit that fixes build of sockmap_basic.c on systems with old
/usr/include fix the build of sockmap_ktls.c

Fixes: d1ba120 ("selftests/bpf: Test unhashing kTLS socket after removing from map")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200219205514.3353788-1-ast@kernel.org

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

@@ -7,6 +7,7 @@
 #include "test_progs.h"
 
 #define MAX_TEST_NAME 80
+#define TCP_ULP 31
 
 static int tcp_server(int family)
 {