Initialize MultiplyingLineView::mCompletionMode member.

Detected by Valgrind:

==6165== Conditional jump or move depends on uninitialised value(s)
==6165==    at 0x880AD5E: KLineEdit::setCompletionMode(KCompletion::CompletionMode) (klineedit.cpp:353)
==6165==    by 0xDFD4843: MessageComposer::RecipientLineNG::setCompletionMode(KCompletion::CompletionMode) (recipientline.cpp:242)
==6165==    by 0x5E24B9A: KPIM::MultiplyingLineView::addLine() (multiplyinglineview_p.cpp:83)
==6165==    by 0x5E22760: KPIM::MultiplyingLineEditor::addData(QSharedPointer<KPIM::MultiplyingLineData> const&) (multiplyinglineeditor.cpp:59)
==6165==    by 0xDFD7346: MessageComposer::RecipientsEditor::RecipientsEditor(QWidget*) (recipientseditor.cpp:99)
==6165==    by 0x52AE615: KMComposerWin::KMComposerWin(QSharedPointer<KMime::Message> const&, bool, bool, KMail::Composer::TemplateContext, unsigned int, QString const&, QString const&) (kmcomposerwin.cpp:323)
==6165==    by 0x52ACE3D: KMComposerWin::create(QSharedPointer<KMime::Message> const&, bool, bool, KMail::Composer::TemplateContext, unsigned int, QString const&, QString const&) (kmcomposerwin.cpp:206)
==6165==    by 0x52ACD90: KMail::makeComposer(QSharedPointer<KMime::Message> const&, bool, bool, KMail::Composer::TemplateContext, unsigned int, QString const&, QString const&) (kmcomposerwin.cpp:199)
==6165==    by 0x5137F8A: KMKernel::recoverDeadLetters() (kmkernel.cpp:1257)
==6165==    by 0x403687: main (main.cpp:148)
==6165==  Uninitialised value was created by a heap allocation
==6165==    at 0x4C2B0D8: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==6165==    by 0x5E22291: KPIM::MultiplyingLineEditor::MultiplyingLineEditor(KPIM::MultiplyingLineFactory*, QWidget*) (multiplyinglineeditor.cpp:37)
==6165==    by 0xDFD7023: MessageComposer::RecipientsEditor::RecipientsEditor(QWidget*) (recipientseditor.cpp:82)
==6165==    by 0x52AE615: KMComposerWin::KMComposerWin(QSharedPointer<KMime::Message> const&, bool, bool, KMail::Composer::TemplateContext, unsigned int, QString const&, QString const&) (kmcomposerwin.cpp:323)
==6165==    by 0x52ACE3D: KMComposerWin::create(QSharedPointer<KMime::Message> const&, bool, bool, KMail::Composer::TemplateContext, unsigned int, QString const&, QString const&) (kmcomposerwin.cpp:206)
==6165==    by 0x52ACD90: KMail::makeComposer(QSharedPointer<KMime::Message> const&, bool, bool, KMail::Composer::TemplateContext, unsigned int, QString const&, QString const&) (kmcomposerwin.cpp:199)
==6165==    by 0x5137F8A: KMKernel::recoverDeadLetters() (kmkernel.cpp:1257)
==6165==    by 0x403687: main (main.cpp:148)