samples: logging/syst: build for C++ too

This extends the samples to build for C++ using the same code.
This shows MIPI Sys-T can work C++ too.

The change to main.c regarding to the struct log_msg_ids is
simply that the compiler errored out complaining the members
must be initialized the same order as the declaration.

Also C++ dislikes a string literal being assigned to char*,
so assign them to const char* instead.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>