Recent glslang update moved a #include inside a #if statement.
The scripts/export_targets.py presubmit check doesn't know how to handle
that so need to exclude the header in the script.

Bug: angleproject:5294
Change-Id: I68563d59983ce2053f9e03c84b2736721db9fe95
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2518175
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>