tools: Compile generate-api-docs into builddir
tools: Compile generate-api-docs into builddir

Since we are building this file every single time, there's no
need for us requiring it to be in srcdir and thus part of a
.tar.gz.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

libvirt/libvirt-php,zippy2/libvirt-php,libvirt/libvirt-php,zippy2/libvirt-php,zippy2/libvirt-php,libvirt/libvirt-php