sysctl: set variable key_sysctls storage-class-specifier to static

smatch reports
security/keys/sysctl.c:12:18: warning: symbol
  'key_sysctls' was not declared. Should it be static?

This variable is only used in its defining file, so it should be static.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>