rpm: unconditionally package rbd-replay-prep(8)

The build system will conditionally build and install the
rbd-replay-prep utility based on the presence of lttng.

Commit 230c5b8e7f58e2e8be32c3f31c6abcc5bdabc5ca adjusted the RPM
packaging so that we only package and ship the rbd-replay-prep utility
and its man page when we're on a platform that has lttng.

The problem is that this is only half-correct. The build system
unconditionally installs the man page for rbd-replay-prep, regardless of
whether lttng was present or absent, and (by extension,) without regard
for whether we've built and installed the rbd-replay-prep binary or not.

We have two options: either the build system should be adjusted to
conditionally skip installing rbd-replay-prep(8), or else we should just
adjust the RPM packaging to always ship the man page. This commit does
the latter.

Backport: firefly
Signed-off-by: Ken Dreyer <kdreyer@redhat.com>