mesa:main: Add flag for EXT_sRGB to gl_extensions

mesa:main: Add flag for EXT_sRGB to gl_extensions

EXT_sRGB is an (incomplete) GLES extension that provides support for sRGB
framebuffer attachments, hence it can be used to check for this support
as an alternative to EXT_framebuffer_sRGB that provies the same
functionality but also sRGB write control support.

However, since EXT_sRGB  is incomplete and superseted by GLES 3.0 it will
not be exposed as an extension.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>