staging: lustre: hide call to Posix ACL in ifdef

A call to forget_cached_acl() was recently added to the lustre file
system, but this is only available when CONFIG_FS_POSIX_ACL is
enabled, otherwise the build now fails with:

lustre/llite/file.c: In function 'll_get_acl':
lustre/llite/file.c:3134:2: error: implicit declaration of function 'forget_cached_acl' [-Werror=implicit-function-declaration]
  forget_cached_acl(inode, type);

This adds one more #ifdef for this call, corresponding to the
other 22 such checks for ACL in lustre.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: b788dc51 ("staging: lustre: llite: drop acl from cache")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>