Move build -> gn/standalone

Make it explicit that those gn files are only for the standalone
build.

Bug: 68710794
Change-Id: I7e2a15219d63b62bf8f249becfdbf39fda2cc183
diff --git a/src/ipc/ipc_library.gni b/src/ipc/ipc_library.gni
index 3a31c73..5231351 100644
--- a/src/ipc/ipc_library.gni
+++ b/src/ipc/ipc_library.gni
@@ -16,7 +16,7 @@
 import("//build_overrides/build.gni")
 
 if (!build_with_chromium) {
-  import("//build/proto_library.gni")
+  import("//gn/standalone/proto_library.gni")
 } else {
   import("//third_party/protobuf/proto_library.gni")
 }