When we use dllexport properly for shared libraries on windows,
there's no longer any issue with linking the object files for
e.g. libavcodec statically into checkasm. (It's still not possible
to link the built object files for e.g. libavformat statically to
libavcodec though, since libavformat exepcts to load av_export_*
symbols from a DLL.)

This reverts commit 4e62b57e.
Signed-off-by: Martin Storsjö <martin@martin.st>