spa: expose utils/cleanup.h for external users

Up until now, `spa/utils/cleanup.h` was not installed,
but 779f06865c ("pod: add spa_auto support for dynamic builder")
included it in a public header. So now `cleanup.h` also needs to
be installed like any other public header so as to not break
3rd party users.

For example, `xdg-desktop-portal-wlr` would break:
https://codesearch.debian.net/search?q=spa_pod_dynamic_builder_init&literal=1

.gitlab/ci/check_missing_headers.sh

@@ -5,7 +5,7 @@
 
 LIST=""
 
-for i in $(find spa/include -name '*.h' -a -not -path 'spa/include/spa/utils/cleanup.h' | sed s#spa/include/##);
+for i in $(find spa/include -name '*.h' | sed s#spa/include/##);
 do
 	[ -f "$PREFIX/include/spa-0.2/$i" ] || LIST="$i $LIST"
 done