Always prefix variables in generated headers with "extern".
Always prefix variables in generated headers with "extern".

Since GCC 10, the linker no longer accepts multiple definitions of the same
variable. We therefore always need to mark the ones in nodes.h as extern.

libfirm/libfirm,libfirm/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm