Add informational message to help with updation of VNDK abi references.

Test: create reference dump for libjpeg; add exported function to libjpeg;
      m -j libjpeg.vendor, build fails with helpful message.

Test: create reference dump for libjpeg; add exported function to libjpeg;
      m -j libjpeg.vendor dist DIST_DIR=dist, build fails with helpful message.

Change-Id: Iae25374fe937a0cbe8a8ddf9e23c3bc1f62bbb2a