Do not use ambigouis %S format character

* src/gpgoladdin.cpp (getCustomUI): Do not use %S.

%S in swprintf used to mean "char *" but at least
in the context of Outlook this now means "wchar_t *"
the wrong interpretation causes undefined behavior
and results in crashes.

The fix is to avoid this altogether as the
rest of the code does and use %hs and %ls instead.