Add F_SEAL_EXEC from Linux 6.3 to bits/fcntl-linux.h.

This patch adds the new F_SEAL_EXEC constant from Linux 6.3 (see Linux
commit 6fd7353829c ("mm/memfd: add F_SEAL_EXEC") to bits/fcntl-linux.h.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>