bpf: fix sysctl.c warning

When CONFIG_BPF_SYSCALL or CONFIG_SYSCTL is disabled, we get
a warning about an unused function:

kernel/sysctl.c:3331:12: error: 'proc_dointvec_minmax_bpf_stats' defined but not used [-Werror=unused-function]
 static int proc_dointvec_minmax_bpf_stats(struct ctl_table *table, int write,

The CONFIG_BPF_SYSCALL check was already handled, but the SYSCTL check
is needed on top.

Fixes: 492ecee8

 ("bpf: enable program stats")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christian Brauner <christian@brauner.io>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>