bpf: Make some symbols static

The sparse tool complains as follows:

kernel/bpf/syscall.c:4567:29: warning:
 symbol 'bpf_sys_bpf_proto' was not declared. Should it be static?
kernel/bpf/syscall.c:4592:29: warning:
 symbol 'bpf_sys_close_proto' was not declared. Should it be static?

This symbol is not used outside of syscall.c, so marks it static.
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210519064116.240536-1-pulehui@huawei.com

kernel/bpf/syscall.c

@@ -4563,7 +4563,7 @@ BPF_CALL_3(bpf_sys_bpf, int, cmd, void *, attr, u32, attr_size)
 	return __sys_bpf(cmd, KERNEL_BPFPTR(attr), attr_size);
 }
 
-const struct bpf_func_proto bpf_sys_bpf_proto = {
+static const struct bpf_func_proto bpf_sys_bpf_proto = {
 	.func		= bpf_sys_bpf,
 	.gpl_only	= false,
 	.ret_type	= RET_INTEGER,
@@ -4588,7 +4588,7 @@ BPF_CALL_1(bpf_sys_close, u32, fd)
 	return close_fd(fd);
 }
 
-const struct bpf_func_proto bpf_sys_close_proto = {
+static const struct bpf_func_proto bpf_sys_close_proto = {
 	.func		= bpf_sys_close,
 	.gpl_only	= false,
 	.ret_type	= RET_INTEGER,