Silence warning about non-void function without return value

Q_UNREACHABLE() isn't enough for some compilers, especially if it
expands to nothing.

warning #1011: missing return statement at end of non-void function "fetchPixel<bpp>(const uchar={unsigned char} *, int) [with bpp=QPixelLayout::BPPNone]"

Change-Id: I3e3f0326f7234a26acf5fffd148fecf0b72ea7e0
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/gui/painting/qdrawhelper.cpp

@@ -843,6 +843,7 @@ template <QPixelLayout::BPP bpp> static
 uint QT_FASTCALL fetchPixel(const uchar *, int)
 {
     Q_UNREACHABLE();
+    return 0;
 }
 
 template <>