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.

(cc'ing @adamcrume in case he wants to review, since he's worked on this area of the packaging recently)