Change shared_sources.gni to use relative path

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2333923003

Review-Url: https://codereview.chromium.org/2333923003
diff --git a/BUILD.gn b/BUILD.gn
index c6841bb..20f8d84 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -3,7 +3,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//gn/shared_sources.gni")
+import("gn/shared_sources.gni")
 
 declare_args() {
   skia_enable_tools = !is_fuchsia && !is_component_build