Makefile: fix fastcheck.

commit 25b7406d11ea3703be864097af66ce95611dde72 tried to make the tests
depend on the info file, but that broke .fast.ok, which used the same
pattern:

	%.ok: $(LINT) %info

This is what happens when you're too tricky!  Simply duplicate the rule,
and change .fast.ok to .fast-ok so it doesn't match both.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>