Removing network.h from matroskadec.c

Removing accidentally added include file.

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
dshow: Fix MSVC support, remove av_export, which was apparently unneeded anyway.

Also cleanup exported symbols

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>