Descend into tracing-* directories when running make

Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>

Author: aspsk

Makefile

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
 
-LESSONS = $(wildcard basic??-*) $(wildcard packet??-*)
+LESSONS = $(wildcard basic??-*) $(wildcard packet??-*) $(wildcard tracing??-*)
 LESSONS_CLEAN = $(addsuffix _clean,$(LESSONS))
 
 .PHONY: clean $(LESSONS) $(LESSONS_CLEAN)