Add missing .clang-format in the named headers directory

The .clang-format was missing in bin/named/include/named, add the
symlink to .clang-format.headers to match the rest of the source tree.

diff --git a/bin/named/include/named/.clang-format b/bin/named/include/named/.clang-format
new file mode 120000 (symlink)
index 0000000..e919bba
--- /dev/null
+++ b/bin/named/include/named/.clang-format
@@ -0,0 +1 @@
+../../../../.clang-format.headers
\ No newline at end of file