Fix mac build

As reported here:
https://groups.google.com/d/msg/android-building/lGawwN0LQP0/KrY194rCCAAJ

WITH_DEXPREOPT already defaults to true on Linux, and is not currently
supported on Mac.

Test: m  (on mac)
Change-Id: Ic3e93fac5a20cce643436406a8e93ff4e28b4f5f
1 file changed