[angle] Include EGL/eglext_angle.h in ANGLE

[hanseuljun]

Describe the pull request
Stops excluding eglext_angle.h from ANGLE. #14890

• What does your PR fix? Fixes #
  eglext_angle.h includes entry points to angle-specific EGL extension functions and this PR exposes them.

• Which triplets are supported/not supported? Have you updated the CI baseline?
  I tested it on x64-windows and arm64-uwp.

• Does your PR follow the maintainer guide?
  I think so. I updated the version of the port at least.

[NancyLi1013]

For the failure on x86-windows, please run vcpkg x-add-version angle and submit the changes to solve it.

Error: While reading versions for port angle from file: C:\a\1\s\versions\a-\angle.json
       Version `2020-05-15-2` was not found in versions file.
       Run:

           vcpkg x-add-version angle

       to add the new port version.

Note: Please update the changes first, then run the above command.

[hanseuljun]

I ran vcpkg x-add-version angle but an error again... I guess it has to do with the previous version being 2020-05-15-1 not 2020-05-15...? Of course it's just a guess...

[NancyLi1013]

Since you have updated versions, need to run vcpkg x-add-version --overwrite-version angle to overwrite the changes.

[hanseuljun]

Ran x-add-version with the overwrite-version flag. Some new hash was assigned so it seems promising!

[ras0219-msft]

LGTM, thanks for the fix!