tools/io_uring: fix compile breakage

It would seem none of the kernel continuous integration does this:
    $ cd tools/io_uring
    $ make

Otherwise it may have noticed:
   cc -Wall -Wextra -g -D_GNU_SOURCE   -c -o io_uring-bench.o
	 io_uring-bench.c
io_uring-bench.c:133:12: error: static declaration of ‘gettid’
	 follows non-static declaration
  133 | static int gettid(void)
      |            ^~~~~~
In file included from /usr/include/unistd.h:1170,
                 from io_uring-bench.c:27:
/usr/include/x86_64-linux-gnu/bits/unistd_ext.h:34:16: note:
	 previous declaration of ‘gettid’ was here
   34 | extern __pid_t gettid (void) __THROW;
      |                ^~~~~~
make: *** [<builtin>: io_uring-bench.o] Error 1

The problem on Ubuntu 20.04 (with lk 5.9.0-rc5) is that unistd.h
already defines gettid(). So prefix the local definition with
"lk_".

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>