tools/io_uring: fix Makefile for pthread library link

tools/io_uring: fix Makefile for pthread library link

Currently fails with:

io_uring-bench.o: In function `main':
/home/axboe/git/linux-block/tools/io_uring/io_uring-bench.c:560: undefined reference to `pthread_create'
/home/axboe/git/linux-block/tools/io_uring/io_uring-bench.c:588: undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status
Makefile:11: recipe for target 'io_uring-bench' failed
make: *** [io_uring-bench] Error 1

Move -lpthread to the end.

Signed-off-by: Jens Axboe <axboe@kernel.dk>