Remove unused JVMFWK_DLLPRIVATE

Change-Id: I9b562e085e0b2a446a16ff5189c1d6b03c0d924b
vendorplugin.h is jvmfwk-internal (and no need for extern "C")

Change-Id: I954f789d5850e8016f5900812f9aa99be2416ce4