win32: don't build vlc.exe for Winstore builds
winvlc.c uses API's not available in UWP:
- SetProcessMitigationPolicy
- SetProcessDEPPolicy
- SetDllDirectory
- SetDefaultDllDirectories
- MessageBox
GetCommandLine is allowed but not in mingw-w64 yet.
Let the build system decide if it can build it. So we can use vlc.exe even when the GUI is not built (meson).