Makefile: Shut up "make clean"

Use `-f` flag of `rm` to avoid error messages from `make clean` when the
targets files don't exist.

Makefile

@@ -69,7 +69,7 @@ manpages += prun/prun.1 prun/pman.1 prun/ptype.1
 all: $(manpages)
 
 clean:
-	@rm $(manpages)
+	@rm -f $(manpages)
 
 install: all
 	@prefix="$(prefix)" usr_prefix="$(usr_prefix)" usrlocal_prefix="$(usrlocal_prefix)" \