apache-httpd/2.4: Fix libxml2 support.

The reason because the configure script is unnable to find libxml2 is because it
is searching for a header file in `libxml/*.h`. Obviously this cases an error,
because it's actually in `${libxml2}/include/libxml2/libxml/*.h`, so let's add
the parent directory to --with-libxml2 and remove the comment from buildInputs.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>