Remove unnecessary header includes from mozcjni.cc (part 2)

This is a follow up CL for r528 (Mozc 2.16.2045.102). Turned out that there remains one unnecessary header include.

This is just a code refactoring, no behavior change is intended.

BUG=none
TEST=none

git-svn-id: https://mozc.googlecode.com/svn/trunk@536 a6090854-d499-a067-5803-1114d4e51264