pkt-line: drop 'const'-ness of a param to set_packet_header()

pkt-line: drop 'const'-ness of a param to set_packet_header()

The function's definition has a paramter of type "int" qualified as
"const".  The fact that the incoming parameter is used as read-only
in the fuction is an implementation detail that the callers should
not have to be told in the prototype declaring it (and "const" there
has no effect, as C passes parameters by value).

The prototype defined for the function in pkt-line.h lacked the
matching "const" for this reason, but apparently some compilers
(e.g. MS Visual C 2017) complain about the parameter type mismatch.

Let's squelch it by removing the "const" that is pointless in the
definition of a small and trivial function like this, which would
not help optimizing compilers nor reading humans that much.

Noticed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>