Makefile: add target for generating tags

Change-Id: Ic2c68c12471d872f449e29082a98df14b47bbda4
diff --git a/Makefile b/Makefile
index f05ab48..87ce759 100644
--- a/Makefile
+++ b/Makefile
@@ -108,3 +108,10 @@
 	$(eval $(call DIR_clean_template,$(dir)))	\
 	$(eval $(call DIR_all_template,$(dir)))		\
 	)
+
+.PHONY: tags
+tags:
+	find $(shell pwd) \
+		-type f -name "*.ttcn" -o \
+		-type f -name "*.ttcnpp" | \
+	xargs ctags