[build] Remove absolute path from openssh-portable build.

Goma does not like absolute paths in the request:
https://screenshot.googleplex.com/C9caBK4goJSXZDr

Change-Id: Iad96b66dce8588b0165ba5c3f90fb2d54363c7df
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/openssh-portable/+/501698
Reviewed-by: Shai Barack <shayba@google.com>
Commit-Queue: Nathan Mulcahey <nmulcahey@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 59783b9..6c1e4d9 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -34,7 +34,7 @@
   ]
   cflags = [
     "-include",
-    rebase_path("fuchsia/fuchsia-compat.h"),
+    rebase_path("fuchsia/fuchsia-compat.h", root_build_dir),
 
     # Suppress warnings in upstream code that are triggered by Fuchsia compilation flags.
     "-Wno-sign-compare",