[gn] Include libcxx configuration file

This was omitted in r359806 but is already referenced in the GN build.

llvm-svn: 359815

utils/gn/secondary/libcxx/config.gni

@@ -0,0 +1,10 @@
+declare_args() {
+  # ABI version of libc++. Can be either 1 or 2, where 2 is currently not stable. Defaults to 1.
+  libcxx_abi_version = 1
+
+  # The inline ABI namespace used by libc++. It defaults to __n where `n` is the current ABI version.
+  libcxx_abi_namespace = ""
+
+  # Unstable ABI of libc++.
+  libcxx_abi_unstable = false
+}