website: Fixed a compile error.

Following the guide step by step in the doc, users will fail on compiling, because DPRINTF must be included before using it.

Change-Id: Ifcd5e68b7b3529c52288aaf02052a286a9e4f5d1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/59213
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
diff --git a/_pages/documentation/learning_gem5/part2/part2_2_debugging.md b/_pages/documentation/learning_gem5/part2/part2_2_debugging.md
index 28aad86..ccc5d48 100644
--- a/_pages/documentation/learning_gem5/part2/part2_2_debugging.md
+++ b/_pages/documentation/learning_gem5/part2/part2_2_debugging.md
@@ -218,6 +218,7 @@
 In the `hello_object.cc` file, we need to include the header file.
 
 ```cpp
+#include "base/trace.hh"
 #include "debug/HelloExample.hh"
 ```