ntdll: Silence a noisy error.

dlls/ntdll/unix/thread.c

@@ -806,7 +806,7 @@ BOOL get_thread_times(int unix_pid, int unix_tid, LARGE_INTEGER *kernel_time, LA
         sprintf( buf, "/proc/%u/task/%u/stat", unix_pid, unix_tid );
     if (!(f = fopen( buf, "r" )))
     {
-        ERR("Failed to open %s: %s\n", buf, strerror(errno));
+        WARN("Failed to open %s: %s\n", buf, strerror(errno));
         return FALSE;
     }