qt: mark as obsolete

qt 4.x is no longer supported upstream since December 2015:

https://blog.qt.io/blog/2015/05/26/qt-4-8-7-released/

So it shouldn't be used for new projects.  Add an obsolete warning to the
package prompt to make this clear.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/qt/Config.in

@@ -3,7 +3,7 @@ comment "qt needs a toolchain w/ C++, threads"
 	depends on !BR2_INSTALL_LIBSTDCPP || !BR2_TOOLCHAIN_HAS_THREADS
 
 menuconfig BR2_PACKAGE_QT
-	bool "Qt"
+	bool "Qt (obsolete)"
 	depends on BR2_USE_MMU # fork
 	depends on BR2_INSTALL_LIBSTDCPP
 	depends on BR2_TOOLCHAIN_HAS_THREADS