Include `<malloc.h>` for `_alloca`

[StephanTLavavej]

Recently, I noticed that MSVC's <exception> was unnecessarily dragging in <malloc.h>, and attempted to remove it in https://github.com/microsoft/STL/pull/4626. This revealed a lot of bugs in open-source projects, so I'll probably revert the change (at least for now). However, I thought I'd report this to you, since it's an easy fix.

asio is using _alloca without including <malloc.h> (the proper header as documented on Microsoft Learn), here:

https://github.com/chriskohlhoff/asio/blob/12e0ce9e0500bf0f247dbd1ae894272656456079/asio/include/asio/detail/impl/socket_ops.ipp#L2542-L2543 https://github.com/chriskohlhoff/asio/blob/12e0ce9e0500bf0f247dbd1ae894272656456079/asio/include/asio/detail/impl/socket_ops.ipp#L2756-L2758

This usage is guarded by #if defined(BOOST_NO_ANSI_APIS) || (defined(_MSC_VER) && (_MSC_VER >= 1800)), but my proposed inclusion is guarded by only #if defined(_MSC_VER) && (_MSC_VER >= 1800) since non-MSVC defined(BOOST_NO_ANSI_APIS) configurations are presumably getting _alloca from somewhere else.