Missing header inclusion for usage of alloca via clang packaged in Visual Studio 2022

[msprotz]

FYI, on the exact combination of: Visual Studio 2022, clang for Windows packaged with Visual Studio, x64, bringing alloca into scope requires including malloc.h (although the error message from clang is incorrect and claims we need to include stdlib.h, not true!).

We already do this for MSVC, so probably need to put an ifdef there too=.