Add C++ tag to header files

[rettichschnidi]

Excerpt from the LLVM Coding Standard:

A few things to note about this particular format: The “-- C++ --” string on the first line is there to tell Emacs that the source file is a C++ file, not a C file (Emacs assumes .h files are C files by default).

Our headers are missing this tag and should be updated accordingly.