um: Include sys/uio.h to have writev()

sys/uio.h gives us writev(), otherwise the build might fail on
some systems.

Fixes: 49da7e64 ("High Performance UML Vector Network Driver")
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/vector_user.c

@@ -30,6 +30,7 @@
 #include <stdlib.h>
 #include <os.h>
 #include <um_malloc.h>
+#include <sys/uio.h>
 #include "vector_user.h"
 
 #define ID_GRE 0