apr-util: workaround reconfigure issue

apr-util: workaround reconfigure issue

PKG_CONFIG_DEPENDS causes a rebuild when needed. Alas the apr-util build
system doesn't always work out the way OpenWrt intends it to. This was
found by pure chance. apr-util was in build directory without any
plugins selected. Then apache got selected, causing plugins to be
selected. So PKG_CONFIG_DEPENDS caused a reconfigure and another make in
apr-util. But afterward apache couldn't find symbols related to LDAP.

To workaround this add "clean" to make targets.

No rev bump because no change for the apr-util packages.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>