drm/i915/selftest: Make guc clients static

Make the private array used for stashing test clients static, to silence
sparse.

References: 55bd6bd75717 ("drm/i915/selftests: Add a GuC doorbells selftest")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171120132606.4254-1-chris@chris-wilson.co.uk
Reviewed-by: Michel Thierry <michel.thierry@intel.com>

drivers/gpu/drm/i915/selftests/intel_guc.c

@@ -27,7 +27,7 @@
 /* max doorbell number + negative test for each client type */
 #define ATTEMPTS (GUC_NUM_DOORBELLS + GUC_CLIENT_PRIORITY_NUM)
 
-struct intel_guc_client *clients[ATTEMPTS];
+static struct intel_guc_client *clients[ATTEMPTS];
 
 static bool available_dbs(struct intel_guc *guc, u32 priority)
 {