Avoid the .stamp dependency in build_generated_jni.mk

This is similar in spirit to commit 1b7fd9a7. This fixes the dependency
of the *_jni.h files so they don't race against the .o generation
anymore.

Bug: 36449525
Test: m -j32
Change-Id: Ia9eccc76a6b8f7e44f0e553fb9c7098358a1c751
1 file changed