debugobjects: Avoid another unused variable warning

debug_objects_maxchecked is only updated in __debug_check_no_obj_freed(),
and only read in debug_objects_maxchecked, unfortunately both of these are
optional and depend on different Kconfig symbols.

When both CONFIG_DEBUG_OBJECTS_FREE and CONFIG_DEBUG_FS are disabled this
warning is emitted:

  lib/debugobjects.c:56:14: error: 'debug_objects_maxchecked' defined but not used [-Werror=unused-variable]

Rather than trying to add more complex #ifdef protections, mark the
variable as __maybe_unused so it can be silently dropped when usused.

Fixes: bd9dcd04

 ("debugobjects: Export max loops counter")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Waiman Long <longman@redhat.com>
Link: https://lkml.kernel.org/r/20180313131857.158876-1-arnd@arndb.de