avcodec/libxvid: add HAVE_UNISTD_H around #include <unistd.h>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
tools/yuvcmp: add HAVE_UNISTD_H around #include <unistd.h>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffserver: add HAVE_UNISTD_H around #include <unistd.h>

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