package/gstreamer/gst-ffmpeg: needs gcc >= 5.x on SuperH

package/gstreamer/gst-ffmpeg: needs gcc >= 5.x on SuperH

Compilation triggers an ICE in gcc with gcc <= 4.9 using this defconfig

BR2_sh=y
BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_PACKAGE_GSTREAMER=y
BR2_PACKAGE_GST_FFMPEG=y
BR2_PACKAGE_GST_FFMPEG_GPL=y

The problem is known upstream, a fix was never committed to gcc <= 4.9:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65151

Compiling gst-ffmpeg with gcc5 works fine using this defconfig:

BR2_sh=y
BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_GCC_VERSION_5_X=y
BR2_PACKAGE_GSTREAMER=y
BR2_PACKAGE_GST_FFMPEG=y
BR2_PACKAGE_GST_FFMPEG_GPL=y

This patch adds a dependency to gcc >= 5.x to fix the problem as
suggested by Thomas:
http://lists.busybox.net/pipermail/buildroot/2016-February/152584.html

Fixes
http://autobuild.buildroot.net/results/939/939da0c7771ddd97c05cedc0a7afc0ad34a21312/

[Peter: fix ML link, simplify kconfig, add comment explaining why]
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>