Include sys/uio.h for writev(2) on FreeBSD

commit 8afbeacfce52d43b7ffb92eead2fccfab7f575ba
Author: Dmitry Marakasov <amdmi3@amdmi3.ru>
Date:   Fri Oct 7 17:19:05 2016 +0300

multicat.c

@@ -44,6 +44,10 @@
 #include <sys/ioctl.h>
 #include <syslog.h>
 
+#ifdef __FreeBSD__
+#   include <sys/uio.h>
+#endif
+
 #ifdef SIOCGSTAMPNS
 #   define HAVE_TIMESTAMPS
 #endif