Specify VMA include relative to source root

This is to help unblock the Skia->Flutter roll after https://skia.googlesource.com/skia.git/+/3b51120a92aa83ab7e465c6437bce23e0c094546.

See also: https://github.com/flutter/flutter/issues/102806#issuecomment-1112998932

Change-Id: I3fd061e495478efccda70d4c3e1647b7c6f375e4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/535356
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
1 file changed