pcap-file: Correctly format enum type.

pcap-file: Correctly format enum type.

The underlying type for an enum is somewhat unpredictable in that the
compiler and the ABI influence it.  The format specifier I used here was
apparently correct for i386 on Linux but wrong for x86-64.  It's better to
just use a cast.

Fixes: 597177a283da ("pcap-file: Add support for Linux SLL formatted PCAP files.")
Reported-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>