xalloc-oversized: relax license to LGPLv2+

xalloc-oversized: relax license to LGPLv2+

Unlike the bulk of xalloc* which calls exit() on allocation failure,
and thus is unacceptable in a library, the xalloc-oversized.h header
is a mere macro that makes checks for allocation overflow possible in
all sorts of portable code.  In fact, the LGPLv2+ module malloca
already has a copy of this check, arguing that this header is more
useful under a permissive license.  See also this thread.
https://lists.gnu.org/archive/html/bug-gnulib/2013-05/msg00092.html

* modules/xalloc-oversized (License): Change from GPLv3+.

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