build: add GN build files

PR-URL: https://github.com/nodejs/node/pull/47637
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>

deps/llhttp/BUILD.gn

@@ -0,0 +1,14 @@
+##############################################################################
+#                                                                            #
+#                       DO NOT EDIT THIS FILE!                               #
+#                                                                            #
+##############################################################################
+
+# This file is used by GN for building, which is NOT the build system used for
+# building official binaries.
+# Please modify the gyp files if you are making changes to build system.
+
+import("unofficial.gni")
+
+llhttp_gn_build("llhttp") {
+}