[dependabot] Update submodule ignore list (#7492)

Fixes the submodule ignore list by using the relative path of the
submodule rather than only the repo name.

Author: pjcollins

.github/dependabot.yml

@@ -14,11 +14,11 @@ updates:
     schedule:
       interval: "daily"
     ignore:
-      - dependency-name: "apksig"
-      - dependency-name: "debugger-libs"
-      - dependency-name: "lz4"
-      - dependency-name: "mman-win32"
-      - dependency-name: "nrefactory"
-      - dependency-name: "opentk"
-      - dependency-name: "robin-map"
-      - dependency-name: "sqlite"
+      - dependency-name: "external/apksig"
+      - dependency-name: "external/debugger-libs"
+      - dependency-name: "external/lz4"
+      - dependency-name: "external/mman-win32"
+      - dependency-name: "external/nrefactory"
+      - dependency-name: "external/opentk"
+      - dependency-name: "external/robin-map"
+      - dependency-name: "external/sqlite"