strtoumax: fix typo in previous commit.

strtoumax: fix typo in previous commit.

Tom G. Christensen reported:

> I see this in the configure output:
> ./configure: line 7680: test: =: unary operator expected
>
> $ sed -n 7680p configure
>   if test $HAVE_STRTOUMAX = 0 || test $REPLACE_STRTOUMAX = 1; then

and indeed, we're only setting HAVE_DECL_STRTOUMAX.

* modules/strtoumax (Depends-on): Fix typo.
* modules/strtoimax (Depends-on): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>