access: wasapi: use the proper signature for DllMain

_DECL_DLLMAIN is needed to get the signature of DllMain. But depending
whether windows.h is included the signature differs (WINAPI, HANDLE, etc).
This is only the case in mingw-w64. In the Windows SDK there's only the
definition without the fancy types.

We need to include process.h before windows.h so we have the same definition
as the Windows SDK.