fix(gen): dedup windows shared-source per-lib copies (LNK2005 ff_open)

FFmpeg's shared sources (file_open.c etc.) are compiled into each lib upstream;
the windows make-n snapshot captured file_open.c in 5 lib dirs, but mcpp builds
one lib → the per-lib copies collide (LNK2005 'ff_open already defined in
file_open.o'). linux/macosx snapshots list each shared source once and build
green. Fix: restrict any OS's shared-source basenames to the dirs the clean
(linux+macosx) reference uses — file_open.c → libavutil only; genuine distinct
same-basename files (libavcodec/4xm.c vs libavformat/4xm.c) stay (different
symbols). windows 2283→2279 sources.

Author: Sunrisepeak