Added diskcache library to known list (#2790)

## Changes
Whitelists diskcache

### Linked issues
Partly resolve #1931

Author: pritishpai

src/databricks/labs/ucx/source_code/known.json

@@ -3712,6 +3712,14 @@
   "dill": {
     "dill": []
   },
+  "diskcache": {
+    "diskcache": [],
+    "diskcache.cli": [],
+    "diskcache.core": [],
+    "diskcache.djangocache": [],
+    "diskcache.persistent": [],
+    "diskcache.recipes": []
+  },
   "distlib": {
     "distlib": [],
     "distlib.compat": [],