nommu: Fix up vmalloc_node() symbol export regression.

Commit e1ca7788 ("mm: add vzalloc() and vzalloc_node() helpers") ended up
accidentally deleting the vmalloc_node() symbol export, resulting in:

"vmalloc_node" [net/core/pktgen.ko] undefined!
"vmalloc_node" [net/netfilter/x_tables.ko] undefined!

regressions.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>