Prevent make distclean removing config/config.rpath

make distclean removes an empty file config/config.rpath.
Avoid that by adding some text.

Also see e1245d83e9("Prevent make distclean removing 0 sized file").

--

1. find . -size 0 ./config/config.rpath
2. ./autogen.sh && ./configure
3. git diff
4. make distclean
5. git diff diff --git a/config/config.rpath b/config/config.rpath deleted file mode 100644 index e69de29bb..000000000

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #8665