Remove default argument from "C" API.

This is far from the only C incompatibility in this file, but it's the
one that's most likely to not break builds, so I'm starting small.

This is a source compat break, but not an ABI break.

Bug: https://github.com/android/ndk/issues/1920
Test: treehugger
Change-Id: I5a32762fa61b1399fb354397479603858184ea05