textproc/randlm: Fix build with Clang 6

In file included from RandLMBuilder.cpp:17:
In file included from ./RandLMBuilder.h:21:
In file included from ./RandLMPipelineTool.h:20:
In file included from ./RandLMPipeline.h:20:
In file included from ./RandLMPreproc.h:25:
In file included from ./RandLM.h:23:
./RandLMQuantiser.h:41:24: error: in-class initializer for static data member of type 'const float' requires 'constexpr' specifier [-Wstatic-float-init]

static const float kFloatErr = 0.00001f;
                   ^           ~~~~~~~~

./RandLMQuantiser.h:41:5: note: add 'constexpr'

static const float kFloatErr = 0.00001f;
^
constexpr

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/randlm-0.2.5_13.log

PR: 230892
Submitted by: tobik
Approved by: demon (maintainer)