Use libvdeplug.h instead of libvdeplug_dyn.h

Use libvdeplug.h instead of libvdeplug_dyn.h

Fix https://github.com/gsliepen/tinc/issues/300

The libvdeplug.h from Debian Unstable is almost identical to the
one from Debian Buster. My making this change, the task of linking
the libvdeplug library is passed to the system dynamic linker at
tincd start time, instead of doing it manually with
libvdeplug_dynopen when vde functionality is actually needed.

This fixes the compilation issue in Ubuntu 21.04 and Debian
Unstable.