io_uring: remove errant ';' from IORING_CQE_F_TSTAMP_HW definition

An errant ';' slipped into that definition, which will cause some
compilers to complain when it's used in an application:

timestamp.c:257:45: error: empty expression statement has no effect; remove unnecessary ';' to silence this warning [-Werror,-Wextra-semi-stmt]
  257 |                 hwts = cqe->flags & IORING_CQE_F_TSTAMP_HW;
      |                                                           ^

Fixes: 9e4ed359 ("io_uring/netcmd: add tx timestamping cmd support")
Signed-off-by: Jens Axboe <axboe@kernel.dk>