dri_util: keep __dri2ConfigOptions symbol private

The symbol was added with commit 45e2b51c853(DRI2/GLX: check for
vblank_mode in DRI2 GLX code) but was never used as such according
to git log.

Possibly it was marked as public due to confusion with
__driConfigOptions which was used for dri1 drivers.

Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

src/mesa/drivers/dri/common/dri_util.c

@@ -52,7 +52,7 @@
 #include "main/errors.h"
 #include "main/macros.h"
 
-PUBLIC const char __dri2ConfigOptions[] =
+const char __dri2ConfigOptions[] =
    DRI_CONF_BEGIN
       DRI_CONF_SECTION_PERFORMANCE
          DRI_CONF_VBLANK_MODE(DRI_CONF_VBLANK_DEF_INTERVAL_1)