repology: exclude package 'dx' from version check

We use value from `dx --version` as package version and Repology
gives false positive update suggestion.

scripts/repology/excluded_packages.txt

@@ -24,6 +24,9 @@ dasm
 # We use version 4.x, 5.x and higher should go to separate package.
 docbook-xml
 
+# Custom versioning (based on output of `dx --version`).
+dx
+
 # Versions higher than 4.12 no longer working.
 ecj