We should include logic for defining EGL prototypes when no other
preference is specified.

Bug: angleproject:3706
Change-Id: Ie1b5ea6150c7c998a9fe71cf5ee7353dca4ae7ba
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1706908
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>