udp_flow: Add missing unistd.h include for close()

For some reason, this is reported only with musl, and older glibc
versions (2.31, at least).

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

udp_flow.c

@@ -8,6 +8,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <sys/uio.h>
+#include <unistd.h>
 
 #include "util.h"
 #include "passt.h"