libavutil: Enable the MSVC DLL symbol loading workaround in shared builds as well

libavutil: Enable the MSVC DLL symbol loading workaround in shared builds as well

This used to only be necessary in static builds (when using the
dynamically linked C runtime), since the _imp prefixed symbols do
exist when linking to the actual DLL. When building testprogs,
however, the current library (e.g. libavutil for some of the testprogs)
is linked statically.

This fixes make fate on DLL builds when using the dynamically
linked C runtime.

Signed-off-by: Martin Storsjö <martin@martin.st>