devel/subversion/Makefile.addons: Add missing .include

Users who incorrectly set WITH_SUBVERSION_VER to something other than
latest or LTS encounter errors.

make -C/usr/ports/devel/py-subversion -VPORTNAME
make: "/usr/ports/Mk/bsd.port.mk" line 1829: Malformed conditional (${ARCH} == "amd64")

in /usr/share/mk/bsd.port.mk:27
in /usr/share/mk/bsd.port.post.mk:4
in /usr/ports/devel/py-subversion/Makefile:34

...

Resolve these errors by adding a missing .include directive.

PR: 281750
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46807