This change 7bbe497a accidentally disabled support
for GL_EXT_buffer_storage extension for the Vulkan
backend, enable it again.

Bug: angleproject:5056
Tests: angle_end2end_tests --gtest_filter=BufferStorageTestES3*Vulkan
Change-Id: If4da2fe2c3e8b098875c89643d366f15fd0ea8b2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2519875
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>