string: Avoid syntax error re strdup in string.in.h (regr. 2021-09-07).
Reported by Jan Engelhardt <jengelh@inai.de>
and by Arash Esbati <arash@gnu.org> via Andy Moreton in
<https://lists.gnu.org/archive/html/bug-gnulib/2021-10/msg00058.html>.
* lib/string.in.h (_GL_ATTRIBUTE_DEALLOC, _GL_ATTRIBUTE_DEALLOC_FREE,
_GL_ATTRIBUTE_MALLOC): Add fallback declarations, like in
lib/stdlib.in.h.
* lib/wchar.in.h (_GL_ATTRIBUTE_DEALLOC, _GL_ATTRIBUTE_DEALLOC_FREE,
_GL_ATTRIBUTE_MALLOC): Likewise.