Export include path from libnativehelper

A lot of libraries depend on headers from this library, but pick them up
because libnativehelper's nativehelper directory is in the global
include path for Android devices.  Take the very first step toward
making these includes explicit by exporting the appropriate include
directory from libnativehelper.

Change-Id: Ie51a42f5d1b9cc12160b710e5ac2f54fd54816a3