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>

kernel/bpf/syscall.c

@@ -5086,6 +5086,14 @@ BPF_CALL_3(bpf_sys_bpf, int, cmd, union bpf_attr *, attr, u32, attr_size)
 	return __sys_bpf(cmd, KERNEL_BPFPTR(attr), attr_size);
 }
 
+
+/* To shut up -Wmissing-prototypes.
+ * This function is used by the kernel light skeleton
+ * to load bpf programs when modules are loaded or during kernel boot.
+ * See tools/lib/bpf/skel_internal.h
+ */
+int kern_sys_bpf(int cmd, union bpf_attr *attr, unsigned int size);
+
 int kern_sys_bpf(int cmd, union bpf_attr *attr, unsigned int size)
 {
 	struct bpf_prog * __maybe_unused prog;