Really Fix Build of rsvg-convert on Windows

Commit 1811f20 attempted to fix the build of rsvg-convert on Windows, but
unfortunately there were some issues about it, as
g_win32_input_stream_new() took a Windows HANDLE (i.e. a void*/gpointer),
not a numeric constant like STDIN_FILENO, as its first argument.

Acquire the HANDLE for stdin on Windows first, and pass it into
g_win32_input_stream_new().  Also use G_OS_WIN32 to check we are building
for Windows, as we do elsewhere for GLib-using items.

https://bugzilla.gnome.org/show_bug.cgi?id=734977