Work around GCC thinking that a variable could be clobbered by longjmp

It can't be. The block in which the variable "s" exists makes no call
to any function that takes the setjmp buffer. This is not a false
positive warning: it's an incorrect warning.

qjpeghandler.cpp:878:6: error: variable ‘s’ might be clobbered by ‘longjmp’ or ‘vfork’ [-Werror=clobbered]

Change-Id: I42e7ef1a481840699a8dffff140681a3d7e34493
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

src/gui/image/qjpeghandler.cpp

@@ -68,6 +68,7 @@ extern "C" {
 }
 
 QT_BEGIN_NAMESPACE
+QT_WARNING_DISABLE_GCC("-Wclobbered")
 
 Q_GUI_EXPORT void QT_FASTCALL qt_convert_rgb888_to_rgb32(quint32 *dst, const uchar *src, int len);
 typedef void (QT_FASTCALL *Rgb888ToRgb32Converter)(quint32 *dst, const uchar *src, int len);