io_uring: Add missing inline to io_uring_cmd_import_fixed() dummy

If CONFIG_IO_URING is not set:

    include/linux/io_uring.h:65:12: error: ‘io_uring_cmd_import_fixed’ defined but not used [-Werror=unused-function]
       65 | static int io_uring_cmd_import_fixed(u64 ubuf, unsigned long len, int rw,
	  |            ^~~~~~~~~~~~~~~~~~~~~~~~~

Fix this by adding the missing "inline" keyword.

Fixes: a9216fac

 ("io_uring: add io_uring_cmd_import_fixed")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/7404b4a696f64e33e5ef3c5bd3754d4f26d13e50.1664887093.git.geert+renesas@glider.be
Signed-off-by: Jens Axboe <axboe@kernel.dk>