ceph.spec.in: quote %files macro in comment

Prior to this commit, RPM would expand the `%files` macro that was present in the comment.

Use a double percent sign to quote the macro so that RPM will not expand it.

This fixes an rpmlint warning, "`W: macro-in-comment %files`"

More information from rpmlint's "`-I`" (help) command:

```
  $ rpmlint -I macro-in-comment
  macro-in-comment:
  There is a unescaped macro after a shell style comment in the
  specfile.  Macros are expanded everywhere, so check if it can cause a
  problem in this case and escape the macro with another leading % if
  appropriate.
```