cgroup: misc: mark dummy misc_cg_res_total_usage() static inline

The dummy implementation was missing static inline triggering the following
compile warning on llvm.

   In file included from arch/x86/kvm/svm/sev.c:17:
>> include/linux/misc_cgroup.h:98:15: warning: no previous prototype for function 'misc_cg_res_total_usage' [-Wmissing-prototypes]
   unsigned long misc_cg_res_total_usage(enum misc_res_type type)
                 ^
   include/linux/misc_cgroup.h:98:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
   unsigned long misc_cg_res_total_usage(enum misc_res_type type)
   ^
   static
   1 warning generated.

Add it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>