ci: Don't use --werror for the win32 build

This leads to warnings from the glib win32 build failing our ci. Only use werror for gtk itself.

Sadly while this makes us overcome the compiler, now the linker is acting up:

warning: subprojects/glib/gobject/libgobject-2.0.dll.a(d000404.o): local symbol `(null)' has no section