qt5base: fix "Unable to find an X11 visual" error

Fix "Unable to find an X11 visual" bug that produce at runtime with Vivante
drivers. Add a conditionnal patch and add variable QT5BASE_EXTRA_CFLAGS in
order to enable workaround.

This patch is actually an improvement over the fix available in Yocto:
https://github.com/Freescale/meta-fsl-arm/blob/dizzy/qt5-layer/recipes-qt/qt5/qtbase/Force_egl_visual_ID_33.patch



[Thomas:
 - Add SoB from Jérôme Pouiller inside the patch imported from Yocto.
 - Add a space between $(TARGET_CFLAGS) and $(QT5BASE_EXTRA_CFLAGS),
   and ditto for CXXFLAGS.
 - Merge the QT5BASE_EXTRA_CFLAGS into an existing
   BR2_PACKAGE_GPU_VIV_BIN_MX6Q conditional.]

Tested-by: Gary Bisson <gary.bisson@boundarydevices.com>
Reviewed-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>