Add '-f ' to 'rm' in 'clean' target.

A tweak to the makefile to avoid this:

```bash
$ make clean
ocamlbuild -clean
rm *~
rm: cannot remove ‘*~’: No such file or directory
makefile:19: recipe for target 'clean' failed
make: [clean] Error 1 (ignored)
rm *.json
rm: cannot remove ‘*.json’: No such file or directory
makefile:19: recipe for target 'clean' failed
make: [clean] Error 1 (ignored)
```