Meson: Add missing flags on Windows

win32_cflags gets used globally as cflags and exposed in the .pc file.
win32_ldflags gets passed to glib-2.0 and exposed in the .pc file.

This should match what the autotools build is currently doing with
GLIB_EXTRA_CFLAGS and G_LIBS_EXTRA.

https://bugzilla.gnome.org/show_bug.cgi?id=784995