Remove 'public' from disabled optional target.

An 'optional' target my wish to define public includes. When the target
is not enabled those public includes should not be present.

Change-Id: I1d0fc4553df100adf9178ad3f57bc0f5bb8f0fcb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/294658
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 15249b9..8c09b76 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -266,6 +266,7 @@
       forward_variables_from(invoker,
                              "*",
                              [
+                               "public",
                                "public_defines",
                                "public_deps",
                                "deps",