build: Overwrite existing symbolic links

build: Overwrite existing symbolic links

The current rule fails if the target already exists:

  cd /home/jenkins/build/libvirt/lib && \
    ln -s libnss_libvirt.so.1 nss_libvirt.so.1
  ln: nss_libvirt.so.1: File exists
  Makefile:3357: recipe for target 'install-exec-hook' failed

However, all other rules concerned with installation are
idempotent and will happily overwrite an existing target,
so this one should as well.