glapi: actually implement GL_EXT_robustness for GLES

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>
(cherry picked from commit 37eee90d)