zbus: fix warning messages when mixing C and C++ files

After some further investigation, I could find the current solution to
enable zbus channels (by using ZBUS_CHAN_DECLARE) to be used in C++
files, which generates a warning when we mix C and C++ code. This fixes
the issue by using an approach to add the extern keyword only when the
file is being compiled with a C++ compiler.

Signed-off-by: Rodrigo Peixoto <rodrigopex@gmail.com>