msvc_recommended_pragmas.h: Treat C4819 warnings as errors

msvc_recommended_pragmas.h: Treat C4819 warnings as errors

Warning C4819 in Visual Studio builds indicates an issue of Visual Studio
2005 and later running on East Asian locales of Windows, which likely
results in broken builds of GLib, Pango, GTK+, and possibly other GNOME
projects such as Cogl and Clutter (and is also an issue when building other
projects like QT and Firefox).

Treat this warning as an error as a result when building GLib-based items
on Visual Studio, and tell people how to remedy this issue correctly.