lsm: make keys for static branch static

lsm: make keys for static branch static

The key use for static-branches are not refrenced by name outside
of the security/security.c file, so make them static. This stops
the sparse warnings about "Should it be static?" such as:

security/security.c: note: in included file:
./include/linux/lsm_hook_defs.h:29:1: warning: symbol
  'security_hook_active_binder_set_context_mgr_0' was not declared.
  Should it be static?
./include/linux/lsm_hook_defs.h:29:1: warning: symbol
  'security_hook_active_binder_set_context_mgr_1' was not declared.
  Should it be static?
...

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
[PM: trimmed sparse output for line-length, readability]
Signed-off-by: Paul Moore <paul@paul-moore.com>