fs: Mark function as static in ext3/xattr_security.c

Mark function as static in ext3/xattr_security.c because it is not used
outside this file.

This eliminates the following warning in ext3/xattr_security.c:
fs/ext3/xattr_security.c:46:5: warning: no previous prototype for ‘ext3_initxattrs’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Jan Kara <jack@suse.cz>