Reduce the number of Makefiles in 'src/' by one more

Reduce the number of Makefiles in 'src/' by one more

The libcommon Makefile was the only one left of the convenience library
Makefiles to ensure that the version header was generated before the
other libraries were built.

By using the 'BUILT_SOURCES' variable we can ensure that 'version.i' is
generated first and merge this in 'src/Makefile'.

Change-Id: I0a6510af65674d1c8bb6a3a08d9a10b07e00fdd7
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11203
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>