Fix linker error in HighlighterEngine test

Complains on highlighter.cpp:213: error: undefined reference to
`Core::MessageManager::write(QString const&,
QFlags<Core::MessageManager::PrintToOutputPaneFlag>)'

Change-Id: I296a105a8322e8b6ac1ab4a7085500f76acfcdeb
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

tests/auto/generichighlighter/highlighterengine/highlighterengine.pro

+QTC_PLUGIN_DEPENDS += coreplugin
 include(../../qttest.pri)
 QT += gui
 PLUGINSDIR = $$IDE_SOURCE_TREE/src/plugins