Update warning message for invalid include

Bug: angleproject:6812
Change-Id: I7f514b192143081acbdcf8d91bf262ede420e0ed
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3340099
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Gregg Tavares <gman@chromium.org>

scripts/export_targets.py

@@ -298,7 +298,7 @@ def has_all_includes(target_name: str, descs: dict) -> bool:
                 #print('  acceptable_sources:')
                 #for x in sorted(acceptable_sources):
                 #    print('   ', x)
-                print('Warning in {}: {}: Invalid include: {}'.format(target_name, cur_file, include), file=sys.stderr)
+                print('Warning in {}: {}: Included file must be listed in the GN target or its public dependency: {}'.format(target_name, cur_file, include), file=sys.stderr)
                 ret = False
             #print('Looks valid:', m.group())
             continue