mingw: Fix symbol export

mingw: Fix symbol export

In commit 98fb83ce25f46e0236706fa6e0943032d921935f I changed the
version script handling. But it seems that I didn't test this properly
and broke it. The .def file is passed to the compiler directly, but it
should get passed to the linker instead.

Set VERSION_SCRIPT_FLAGS to -Wl, to pass the .def file correctly to
the linker.

This fixes the undefined symbol errors while linking virsh.