[build] Forward debian_package target.

The SDK builders will make use of this target.

Bug: #26953
Change-Id: I47e2f6fe2a95cb9edbba4fd1152c511a0d4f0271
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/429441
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: rmacnak-google

BUILD.gn

@@ -491,3 +491,9 @@ if (is_fuchsia) {
     }
   }
 }
+
+if (is_linux) {
+  group("debian_package") {
+    public_deps = [ "tools/debian_package" ]
+  }
+}