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 microsoft/STL#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:

asio/asio/include/asio/detail/impl/socket_ops.ipp

Lines 2542 to 2543 in 12e0ce9

	#if defined(BOOST_NO_ANSI_APIS) || (defined(_MSC_VER) && (_MSC_VER >= 1800))
	LPWSTR string_buffer = (LPWSTR)_alloca(length * sizeof(WCHAR));

asio/asio/include/asio/detail/impl/socket_ops.ipp

Lines 2756 to 2758 in 12e0ce9

	#if defined(BOOST_NO_ANSI_APIS) || (defined(_MSC_VER) && (_MSC_VER >= 1800))
	int num_wide_chars = static_cast<int>(strlen(src)) + 1;
	LPWSTR wide_buffer = (LPWSTR)_alloca(num_wide_chars * sizeof(WCHAR));

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.