...which mentions extern "C", so CppUnit's TestPlugInSignature must reflect
that. It is a bit odd how CPPUNIT_PLUGIN_EXPORT needs to be passed into
CppUnit, so probably best to keep this as a local patch for now. (Clang's
-fsanitize=undefined complained about the mismatch.)
Change-Id: Ied179a1afe82ceb04de4739c14cf8fadff31b80f
