wchar: add explicit dependencies, for Tru64

wchar: add explicit dependencies, for Tru64

Commit 9a354cd exposed a few modules that were previously
picking up the <wchar.h> replacement via indirect dependency,
rather than explicitly including it.  This led to a compile
failure on Tru64 for the vsnprintf module, for example.

* modules/mbmemcasecoll (Depends-on): Add wchar.
* modules/mbtowc (Depends-on): Likewise.
* modules/vasnprintf (Depends-on): Likewise.
* modules/unistdio/u-printf-args (Depends-on): Likewise.
* modules/wctomb (Depends-on): Likewise.
Reported by Peter O'Gorman.

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