apparmor: Mark alloc_unconfined() as static

Kernel test robot throws below warning ->
security/apparmor/policy_ns.c:83:20: warning: no previous prototype
for function 'alloc_unconfined' [-Wmissing-prototypes]

Mark it as static.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Souptick Joarder (HPE) <jrdr.linux@gmail.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>