emulators/kcemu: Fix build with Clang 6

n file included from plotter.cc:26:
../../../include/kc/plotter.h:31:23: error: in-class initializer for static data member of type 'const double' requires 'constexpr' specifier [-Wstatic-float-init]

static const double WIDTH_MM = 210.0;
                    ^          ~~~~~

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p479076_s338486/logs/errors/kcemu-0.5.1_15.log

PR: 230960
Approved by: hardy.schumacher@gmx.de (maintainer timeout, 2 weeks)