selftests/net: update initializer syntax to use c99 designators

Before, clang version 9 threw errors such as: error:
use of GNU old-style field designator extension [-Werror,-Wgnu-designator]
                { tstamp: true, swtstamp: true }
                  ^~~~~~~
                  .tstamp =
Fix these warnings in tools/testing/selftests/net in the same manner as
commit 121e357a

 ("selftests/harness: Update named initializer syntax").
N.B. rxtimestamp.c is the only affected file in the directory.

Signed-off-by: Tanner Love <tannerlove@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>