io_uring: fix !CONFIG_BLOCK compilation failure

kernel test robot correctly pinpoints a compilation failure if
CONFIG_BLOCK isn't set:

fs/io_uring.c: In function '__io_complete_rw':
>> fs/io_uring.c:2509:48: error: implicit declaration of function 'io_rw_should_reissue'; did you mean 'io_rw_reissue'? [-Werror=implicit-function-declaration]
    2509 |  if ((res == -EAGAIN || res == -EOPNOTSUPP) && io_rw_should_reissue(req)) {
         |                                                ^~~~~~~~~~~~~~~~~~~~
         |                                                io_rw_reissue
    cc1: some warnings being treated as errors

Ensure that we have a stub declaration of io_rw_should_reissue() for
!CONFIG_BLOCK.

Fixes: 230d50d4 ("io_uring: move reissue into regular IO path")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2479,6 +2479,11 @@ static bool io_rw_should_reissue(struct io_kiocb *req)
 		return false;
 	return true;
 }
+#else
+static bool io_rw_should_reissue(struct io_kiocb *req)
+{
+	return false;
+}
 #endif
 
 static bool io_rw_reissue(struct io_kiocb *req)