audit: Move audit_log_task declaration under CONFIG_AUDITSYSCALL

The 0-DAY found that audit_log_task is not declared under
CONFIG_AUDITSYSCALL which causes compilation error when
it is not defined:

    kernel/bpf/syscall.o: In function `bpf_audit_prog.isra.30':
 >> syscall.c:(.text+0x860): undefined reference to `audit_log_task'

Adding the audit_log_task declaration and stub within
CONFIG_AUDITSYSCALL ifdef.

Fixes: 91e6015b

 ("bpf: Emit audit messages upon successful prog load and unload")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>