Remove residual blank line at start of Makefile

This is a residual of the $FreeBSD$ removal.

MFC After: 3 days (though I'll just run the command on the branches)
Sponsored by: Netflix

pflow: import from OpenBSD

pflow is a pseudo device to export flow accounting data over UDP.
It's compatible with netflow version 5 and IPFIX (10).

The data is extracted from the pf state table. St

pflow: import from OpenBSD

pflow is a pseudo device to export flow accounting data over UDP.
It's compatible with netflow version 5 and IPFIX (10).

The data is extracted from the pf state table. States are exported once
they are removed.

Reviewed by: melifaro
Obtained from: OpenBSD
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D43106

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