fdo#70393: move libxmlsec to a subdir of external

Change-Id: I1bcdd01aad7fc2ee2d2f635b0ae4c4183c9ab092
Reviewed-on: https://gerrit.libreoffice.org/6571
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>