sysctl: Fix unused function warning

The newly added bpf_stats_handler function has the wrong #ifdef
check around it, leading to an unused-function warning when
CONFIG_SYSCTL is disabled:

kernel/sysctl.c:205:12: error: unused function 'bpf_stats_handler' [-Werror,-Wunused-function]
static int bpf_stats_handler(struct ctl_table *table, int write,

Fix the check to match the reference.

Fixes: d46edd671a ("bpf: Sharing bpf runtime stats with BPF_ENABLE_STATS")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200505140734.503701-1-arnd@arndb.de

kernel/sysctl.c

@@ -201,7 +201,7 @@ static int max_extfrag_threshold = 1000;
 
 #endif /* CONFIG_SYSCTL */
 
-#ifdef CONFIG_BPF_SYSCALL
+#if defined(CONFIG_BPF_SYSCALL) && defined(CONFIG_SYSCTL)
 static int bpf_stats_handler(struct ctl_table *table, int write,
 			     void __user *buffer, size_t *lenp,
 			     loff_t *ppos)