selftest: fix headers in fclog.c

selftest: fix headers in fclog.c

fclog.c does not compile because it is missing fcntl.h, needed for
O_RDONLY etc.

There are also some redundant includes that are also in
kselftest_harness.h.

Link: https://lore.kernel.org/20260710171741.837308-1-jkoolstra@xs4all.nl
Signed-off-by: Jori Koolstra <jkoolstra@xs4all.nl>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>