Fix some compiler warnings.

As it turns out, we had an overloaded virtual from an earlier era,
with the extra parameter never used.  So cleaning that up was a
bonus to remove the compiler warning.

Change-Id: I780287f8a5d2b0a1ec84ec62c88ba50e051f372b
Reviewed-by: Jonas Rabbe <jonas.rabbe@nokia.com>

examples/spectrum/app/waveform.cpp

@@ -393,6 +393,7 @@ void Waveform::paintTile(int index)
         const int offset = reinterpret_cast<const char*>(ptr) - m_buffer.constData();
         Q_ASSERT(offset >= 0);
         Q_ASSERT(offset < m_bufferLength);
+        Q_UNUSED(offset);
 
         const qint16 pcmValue = *ptr;
         const qreal realValue = pcmToReal(pcmValue);