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

spa/include/meson.build

@@ -15,7 +15,4 @@ spa_sections = [
 spa_headers = 'spa'  # used by doxygen
 install_subdir('spa',
   install_dir : get_option('includedir') / spa_name,
-  exclude_files : [
-    'utils/cleanup.h',
-  ],
 )