Remove plugin_common library from MSVC build

Previously src/plugin_common library was used by FLAC's own Winamp plugin
(MSVC/Windows) and by the XMMS plugin (*nix). The Winamp plugin is long
gone from FLAC tree, so plugin_common is unused on Windows.

Patch-from: lvqcl <lvqcl.mail@gmail.com>
6 files changed