The extension was exposed but not the functions.
This fixes:
dEQP-GLES31.functional.debug.negative_coverage.get_error.buffer.readn_pixels
dEQP-GLES31.functional.debug.negative_coverage.get_error.state.get_nuniformfv
dEQP-GLES31.functional.debug.negative_coverage.get_error.state.get_nuniformiv
Cc: 18.1 18.2 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>