Add GL_EXT_sRGB_write_control in registry_xml.py.
This makes FrameCapture write a GL_FRAMEBUFFER_SRGB_EXT
enum instead of it's hex value in glEnable/Disable calls.

Bug: angleproject:5857
Change-Id: Ifa01118c2958275c7623c6bfe29f015fa8875fce
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2988356Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Lubosz Sarnecki <lubosz.sarnecki@collabora.com>