Add more include dirs to find curlbuild.h

curlbuild.h is included as both "curlbuild.h" and <curl/curlbuild.h>.
Since it is normally produced into the source directory, both work for
headers in curl/ as the ""-style includes have the current directory
implicitly in the path.. In the Fuchsia build, since we don't put this
into the source directory, we have to add both include dirs to the path.

This fixes clean builds (I locally had curlbuild.h in both locations,
so didn't see this error).

Change-Id: If5f2aac160b221f513d3b113e8604b99071e04e5
diff --git a/BUILD.gn b/BUILD.gn
index 13133e4..3767f07 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -30,6 +30,7 @@
   include_dirs = [
     "include",
     "$target_out_dir",
+    "$target_out_dir/curl",
   ]
 }