mesa/st: Add support for EXT_texture_sRGB_R8

mesa/st: Add support for EXT_texture_sRGB_R8

This only adds support on the Gallium core level, for the drivers
it is likely that additional changes are needed to support the
new texture format and thereby enabling the extension.

Enables on softpipe and makes pass:
  dEQP-GLES31.functional.srgb_texture_decode.skip_decode.sr8.*

v2: - add include for getting GL_SR8_EXT
v4: - since the extension is not required don't bother providing
      a fallback (Ilia Mirkin)
    - split patch (2/2) to separate Gallium and mesa/st parts
      (Roland Scheidegger)
    - trim commit message to only contain the history of the patch
      relevant to this part
v5: - don't include GLES headers (required enum has been added to glheader.h)
      (Ilia Mirkin)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>