main/rdiff-backup: fix version reported as 0.0.0

Before:
 $ rdiff-backup --version
 rdiff-backup 0.0.0
(cherry picked from commit bc4c8c28)
8 jobs for !25558 with v3.14-rdiff-backup-fix-version in 9 minutes and 52 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Lint
passed #492292
docker-alpine x86_64
lint

00:00:23

 
  Build
passed #492297
docker-alpine ci-build aarch64
build-aarch64

00:01:48

passed #492299
docker-alpine ci-build armhf
build-armhf

00:01:55

passed #492298
docker-alpine ci-build armv7
build-armv7

00:02:33

passed #492296
docker-alpine ci-build ppc64le
build-ppc64le

00:01:20

passed #492295
docker-alpine ci-build s390x
build-s390x

00:00:44

passed #492294
docker-alpine ci-build x86
build-x86

00:00:38

passed #492293
docker-alpine ci-build x86_64
build-x86_64

00:00:29