`win32_ucontext.h`, included by `julia.h` no longer available.

[KristofferC]

PackageCompiler CI started erroring with:

In file included from D:\a\PackageCompiler.jl\PackageCompiler.jl\src\embedding_wrapper.c:10:
C:\hostedtoolcache\windows\julia\nightly\x64\include\julia/julia.h:42:12: fatal error: win32_ucontext.h: No such file or directory
 #  include "win32_ucontext.h"
            ^~~~~~~~~~~~~~~~~~
compilation terminated.

on nightly. This file was moved in #41936 from src/support/win32_ucontext.h to src/win32_ucontext.h.

[JeffBezanson]

Ah, looks like it needs to be in PUBLIC_HEADERS?