Bug 16222 - Add REST API folder to Makefile.PL

Bug 16222 - Add REST API folder to Makefile.PL

Add `api` folder to INTRANET_CGI_FOLDER in Makefile.PL so that the new
RESTFul API introduced in bug #13799 works also when installing with make.

Test plan:

- apply patch and do a `perl Makefile.PL` install
- observe that INSTALL_DIR/intranet/cgi-bin now has a folder `api`
  containing the swagger files and the api cgi mount, app.pl.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.z>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher <bredan@bywatersolutions.com>
(cherry picked from commit c20c70c99d79c0d02a37a0e41d0e90afb5c69681)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>