[werror] Fix c++11-compat-reserved-user-defined-literal

c++11 readiness, avoid clash with
http://www.stroustrup.com/C++11FAQ.html#UD-literals

Signed-off-by: Thorsten Behrens <tbehrens@suse.com>
Merge pull request #2066 from thorstenb/wip-janitorial-clang-2

[werror] Fix c++11-compat-reserved-user-defined-literal