sched/topology: Move sd_flag_debug out of #ifdef CONFIG_SYSCTL

The last sd_flag_debug shuffle inadvertently moved its definition within
an #ifdef CONFIG_SYSCTL region. While CONFIG_SYSCTL is indeed required to
produce the sched domain ctl interface (which uses sd_flag_debug to output
flag names), it isn't required to run any assertion on the sched_domain
hierarchy itself.

Move the definition of sd_flag_debug to a CONFIG_SCHED_DEBUG region of
topology.c.

Now at long last we have:

- sd_flag_debug declared in include/linux/sched/topology.h iff
  CONFIG_SCHED_DEBUG=y
- sd_flag_debug defined in kernel/sched/topology.c, conditioned by:
  - CONFIG_SCHED_DEBUG, with an explicit #ifdef block
  - CONFIG_SMP, as a requirement to compile topology.c

With this change, all symbols pertaining to SD flag metadata (with the
exception of __SD_FLAG_CNT) are now defined exclusively within topology.c

Fixes: 8fca9494

 ("sched/topology: Move sd_flag_debug out of linux/sched/topology.h")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200908184956.23369-1-valentin.schneider@arm.com