Prevent `make distclean` removing config/config.rpath

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").

--
 # find . -size 0
 ./config/config.rpath
 # ./autogen.sh && ./configure
 # git diff
 # make distclean
 # 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