src/Makefile.am: add libcpp-common convenience library

src/Makefile.am: add libcpp-common convenience library

Some following commits are expected to add .cpp files to the cpp-common
library.  Turn it into a convenience libtool library.

Add a temporary dummy.cpp file, since some platforms don't like building
a library without at least one compiled source file.

Change-Id: I702b242ca2d229ab5064ba36d422368fd9a03909
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11715
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>