mksh: fix ulimit.

In the absence of the correct #define, mksh was falling back to a trivial
internal list of limits (that doubly annoyingly wasn't reflected in the
usage output).

Bug: http://b/281900697
Test: ulimit -a
(cherry picked from https://android-review.googlesource.com/q/commit:3c75470b2730985cfd83a4c0d83a3f7e81e7c2f8)
Merged-In: I89a157db76cd69c2cc2c9e4a8a8c67364a7a6c4b
Change-Id: I89a157db76cd69c2cc2c9e4a8a8c67364a7a6c4b