Fixed NetFlow v9 flow data misalignment

NetFlow v9 flow data has to be aligned on a 32 bit boundary by inserting padding at the end of the flowset.
In pfSense 2.0, every even flow in a NetFlow v9 flowset is shifted by two bytes. Those extra bytes are added by the compiler, which aligns NF9_IPV4_DATA on a 4-byte boundary (42 bytes -> 44 bytes)

Adding "packed" attribute fixed this issue.
Verified using Wireshark and Plixer Scrutinizer.