powercap: DTPM: dtpm_node_callback[] can be static

drivers/powercap/dtpm.c:525:22: warning: symbol 'dtpm_node_callback' was not declared. Should it be static?

Fixes: 3759ec67 ("powercap/drivers/dtpm: Add hierarchy creation")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: kernel test robot <lkp@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>