linux: fix name of config variable

linux: fix name of config variable

In 4c10eedc1 (systemd: enable required kernel features), we added
setting a few required kernel features to ensure systemd works.

However, there was a typo for one of the variables: CONFIG_TMPFS_XATTR
was written as CONFIG_TMPFS_POSIX_XATTR, which does not exist (and never
ever existed, at least not since 2.6.12)...

Reported-by: Michael Heinemann <posted@heine.so>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Cc: Michael Heinemann <posted@heine.so>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>