cmutils: include shellapi.h on Win32 (for CommandLineToArgvW).

This is required for CommandLineToArgvW. Normally, shellapi.h is included
implicitly by windows.h, but if we define WIN32_LEAN_AND_MEAN (or some of
the other earlier headers have included windows.h with that option),
windows.h doesn't include this one.

Thus explicitly include the headers we really need, for clarity and
compatibility.