drm/xe: make xe_pm_runtime_lockdep_map a static struct

Fix the new sparse warning:

>> drivers/gpu/drm/xe/xe_pm.c:72:20: sparse: sparse: symbol
'xe_pm_runtime_lockdep_map' was not declared. Should it be static?

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202404191329.EZzOTzwK-lkp@intel.com/
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Badal Nilawar <badal.nilawar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240422201454.699089-1-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_pm.c

@@ -69,7 +69,7 @@
  */
 
 #ifdef CONFIG_LOCKDEP
-struct lockdep_map xe_pm_runtime_lockdep_map = {
+static struct lockdep_map xe_pm_runtime_lockdep_map = {
 	.name = "xe_pm_runtime_lockdep_map"
 };
 #endif