build: fix building embedtest in GN build

PR-URL: https://github.com/nodejs/node/pull/53145
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

unofficial.gni

@@ -346,6 +346,7 @@ template("node_gn_build") {
     output_name = "embedtest"
     testonly = true
     deps = [ ":libnode" ]
+    include_dirs = [ "tools" ]
     sources = [
       "src/node_snapshot_stub.cc",
       "test/embedding/embedtest.cc",