Add a target for the cscope index

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/Makefile b/Makefile
index a6ee2ae..663df86 100644
--- a/Makefile
+++ b/Makefile
@@ -144,3 +144,5 @@
 	etags -o $@ $(C_SOURCE_FILES)
 GPATH GRTAGS GSYMS GTAGS: $(C_SOURCE_FILES)
 	ls $(C_SOURCE_FILES) | gtags -f - --gtagsconf .globalrc
+cscope.in.out cscope.po.out cscope.out: $(C_SOURCE_FILES)
+	cscope -bq -u -Iinclude -Ilibrary $(patsubst %,-I%,$(wildcard 3rdparty/*/include)) -Itests/include $(C_SOURCE_FILES)