MSVC Builds: Don't Include the Dropped Loaders

The PCX, WBMP and RAS loaders have been dropped, so don't include them.
They should be dropped from gdk-pixbuf-io.c as well, otherwise if someone
is building with INCLUDE_xxx for them will get linker errors.

https://bugzilla.gnome.org/show_bug.cgi?id=721372