util: add missing DRI_CONF_OPT_END

When DRI_CONF_GLES_EMULATE_BGRA was added for the virgl driver, it missed a DRI_CONF_OPT_END.

This make some drivers, like v4c/v3d to crash with the following error: Fatal error in __driConfigOptions line 99, column 2: mismatched tag.

Not sure why it doesn't fail with virgl.

Fixes: b7936634

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