FindXMLRPC: Tolerate no include directories

`xmlrpc-c-config $modules --cflags` may report no `-I` flags if the
headers are in a standard location like `/usr/include`.  In this case
it is okay for `XMLRPC_INCLUDE_DIRS` to be empty.

Ideally the `--cflags` output should be used as hints for a `find_path`
call to really find the headers as we do for other find modules, but
simply assuming no explicit include directories are needed is good
enough for now.

Fixes: #17347