Correct C++-style comment in header file (-pedantic).

Ironically, the comment occurs before we make sure that we are included from a C++ file ;-)

Strange this hasn't occurred before. I set the -pedantic switch yesterday mainly because I realized it's set for gcc on Travis, and it promptly complained about this line. 

Ah, now I realize why. I compiled with Code::Blocks, which cannot easily differentiate between C and C++ where it's command line is concerned. So this file gets #included for C files as well.

In any case, the safe thing is to use a C-style comment here.