fix: pass exec-prefix to python bindings install

fix: pass exec-prefix to python bindings install

Some systems will split the arch independent files from the arch
specific using a different value for 'prefix' and 'exec-prefix'. Pass
this information from the Makefile to the python build system.

Change-Id: I2f26ba1d8c8252e5a1c49fb54733e02ae336f157
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/9695
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>