Prevent make distclean removing config/config.rpath

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

Also see e1245d8("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 <[email protected]>
Signed-off-by: Tomohiro Kusumi <[email protected]>
Closes openzfs#8665

config/config.rpath

@@ -0,0 +1 @@
+# `make distclean` deletes files with size 0. This text is to avoid that.