Fix “unused variable” warning

Change-Id: I2b151d7a0cda8774c236de1a024acbb3a1875c7e
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/gui/painting/qimagescale.cpp

@@ -322,6 +322,8 @@ static inline void multithread_pixels_function(QImageScaleInfo *isi, int dh, con
         semaphore.acquire(segments);
         return;
     }
+#else
+    Q_UNUSED(isi);
 #endif
     scaleSection(0, dh);
 }