kernel: allow building zsmalloc as a module again

zsmalloc was changed to a bool because of missing exports in the kernel,
but we already export the required symbol, so change it back to tristate.

Closes #13481.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@36587 3c298f89-4303-0410-b956-a3cf2f4a3e73