Make dirmd available in recipe bundle

`dirmd` was not previously included in the recipes bundle, and couldn't
be used in recipes. Update `.gitattributes` to include it in the bundle.

Its dependencies should already be covered by `/*.bat`, `/*.sh`,
and `/cipd_manifest*` patterns.

Bug: 416037494
Change-Id: I320cb6ef9b5f1679c95c4f36ece534ddc75bcdde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6546185
Reviewed-by: Scott Lee <ddoman@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>

.gitattributes

@@ -16,6 +16,7 @@
 # Extensionless tools we want and support scripts.
 /autoninja*                   recipes
 /clang*                       recipes
+/dirmd                        recipes
 /download_from_google_storage recipes
 /fetch                        recipes
 /gclient                      recipes