[Minor] Make ragel-generated files independent of the parent directory

[Minor] Make ragel-generated files independent of the parent directory

* Use relative paths for `#line` directives to allow reproducible builds
  independent of the absolute build directory. For /b/rspamd/src/x.rl
  and build dir /b/build, it could result in `../../build/src/x.rl`.
* The `-L` option for `Inhibit writing #line directives` is useless, it
  just comments out the line with `/* ... */`, but that means that the
  source file is still different.
* Note that ragel only accepts one input file, despite plural `INPUTS`.