mm: export "high_memory" symbol on !MMU

mm: export "high_memory" symbol on !MMU

The symbol 'high_memory' is provided on both MMU- and NOMMU-kernels, but
only one of them is exported, which leads to module build errors in
drivers that work fine built-in:

  ERROR: "high_memory" [drivers/net/virtio_net.ko] undefined!
  ERROR: "high_memory" [drivers/net/ppp/ppp_mppe.ko] undefined!
  ERROR: "high_memory" [drivers/mtd/nand/nand.ko] undefined!
  ERROR: "high_memory" [crypto/tcrypt.ko] undefined!
  ERROR: "high_memory" [crypto/cts.ko] undefined!

This exports the symbol to get these to work on NOMMU as well.

Signed-off-by: Arnd Bergmann <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Acked-by: Greg Ungerer <[email protected]>
Acked-by: David Rientjes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>