bt2: put SWIG interface file C code in separate files

bt2: put SWIG interface file C code in separate files

This patch moves any code in `%{ %}` blocks within SWIG interface files
(`.i`) to their own files. For a SWIG interface file named `file.i`, the
code is moved to `file.i.h`. In `file.i`, there is:

    %{
    #include "file.i.h"
    %}

This helps with text editors which do not always know how to highlight
the code of a `.i` file.

In `Makefile.am`, the `.i.h` files are added to the
`SWIG_INTERFACE_FILES` variable as they can be considered part of the
SWIG interface files.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I28975c9b3c827e65ba77af9134521ce1b920d740
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1812
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>