Broadway: Allow Compilation on Windows/MSVC

-Don't include unistd.h unconditionally as it's not available in Visual
 Studio, but include io.h where necessary.
-Avoid C99isms, and use _chsize_s in place of ftruncate when unistd.h is
 not available (as in the case of Visual Studio)

gdk/broadway/gdkbroadway-server.c

@@ -18,7 +18,12 @@
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
+#ifdef HAVE_UNISTD_H
 #include <unistd.h>
+#elif defined (G_OS_WIN32)
+#include <io.h>
+#define ftruncate _chsize_s
+#endif
 #include <sys/types.h>
 #ifdef G_OS_WIN32
 #include <windows.h>