lib/Kconfig.debug: TEST_IOV_ITER depends on MMU

Trying to run the iov_iter unit test on a nommu system such as the qemu
kc705-nommu emulation results in a crash.

KTAP version 1
# Subtest: iov_iter
# module: kunit_iov_iter
1..9
BUG: failure at mm/nommu.c:318/vmap()!
Kernel panic - not syncing: BUG!

The test calls vmap() directly, but vmap() is not supported on nommu
systems, causing the crash. TEST_IOV_ITER therefore needs to depend on
MMU.

Link: https://lkml.kernel.org/r/20240208153010.1439753-1-linux@roeck-us.net
Fixes: 2d71340ff1d4 ("iov_iter: Kunit tests for copying to/from an iterator")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: David Howells <dhowells@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>