bpf: Shut up kern_sys_bpf warning.

Shut up this warning:
kernel/bpf/syscall.c:5089:5: warning: no previous prototype for function 'kern_sys_bpf' [-Wmissing-prototypes]
int kern_sys_bpf(int cmd, union bpf_attr *attr, unsigned int size)

Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>