pf tests: pfsync:defer requires scapy

Add the kyua hint so we skip the test if it's not installed.

Sponsored by:	Rubicon Communications, LLC ("Netgate")

tests/sys/netpfil/pf/pfsync.sh

@@ -118,6 +118,7 @@ defer_head()
 {
 	atf_set descr 'Defer mode pfsync test'
 	atf_set require.user root
+	atf_set require.progs scapy
 }
 
 defer_body()