build: add cache to distributed files

We were missing the cache directory on the distributed files.
diff --git a/Makefile.am b/Makefile.am
index a1b6632..50ce1d4 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -267,6 +267,7 @@
 
 
 EXTRA_DIST += \
+	testsuite/module-playground/cache \
 	testsuite/module-playground/dummy.sha1 \
 	testsuite/module-playground/dummy.sha256 \
 	testsuite/module-playground/Makefile \