MAINTAINERS: repair file entry in SECURITY SUBSYSTEM

Commit 67889688 ("MAINTAINERS: update the LSM file list") adds a few
file entries to lsm-related header files. Among them, there is a reference
to include/security.h. However, security.h is located in include/linux/,
not in include/.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains about a
broken reference.

Repair this new file entry in the SECURITY SUBSYSTEM section.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>