Addition of just the entry points for GL_EXT_buffer_storage extension.

Bug: angleproject:5056
Change-Id: I7a3d7c151f004f89fd945d5d06dbe7afcd491578
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2419951
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>