x86/speculation: Add missing prototype for unpriv_ebpf_notify()

Fix the following warnings seen with "make W=1":

  kernel/sysctl.c:183:13: warning: no previous prototype for ‘unpriv_ebpf_notify’ [-Wmissing-prototypes]
    183 | void __weak unpriv_ebpf_notify(int new_state)
        |             ^~~~~~~~~~~~~~~~~~

  arch/x86/kernel/cpu/bugs.c:659:6: warning: no previous prototype for ‘unpriv_ebpf_notify’ [-Wmissing-prototypes]
    659 | void unpriv_ebpf_notify(int new_state)
        |      ^~~~~~~~~~~~~~~~~~

Fixes: 44a3918c ("x86/speculation: Include unprivileged eBPF status in Spectre v2 mitigation reporting")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/5689d065f739602ececaee1e05e68b8644009608.1650930000.git.jpoimboe@redhat.com

include/linux/bpf.h

@@ -2085,6 +2085,8 @@ void bpf_offload_dev_netdev_unregister(struct bpf_offload_dev *offdev,
 				       struct net_device *netdev);
 bool bpf_offload_dev_match(struct bpf_prog *prog, struct net_device *netdev);
 
+void unpriv_ebpf_notify(int new_state);
+
 #if defined(CONFIG_NET) && defined(CONFIG_BPF_SYSCALL)
 int bpf_prog_offload_init(struct bpf_prog *prog, union bpf_attr *attr);