Conflict when including netinet/if_ether.h before of13match.hh

[fabiomargarido]

On Linux (x86_64, kernel 3.19), the header file netinet/if_ether.h defines a macro arp_op. When this file is included before of13match.hh, the macro conflicts with the definition of ARPOp *arp_op(); in the libfluid header, causing the following compilation error:

/usr/local/include/fluid/of13/of13match.hh:1277:12: error: expected ‘;’ at end of member declaration
     ARPOp* arp_op();
            ^
/usr/local/include/fluid/of13/of13match.hh:1277:12: error: expected unqualified-id before ‘.’ token
     ARPOp* arp_op();
            ^