ci(coverity): annotate register_cfunc as leaking memory

register_cfunc allocates a ufunc_T, but doesn't store the pointer
anywhere before returning.  The uf_name member variable is stored in a
hashtable and used to lookup the ufunc_T later, but that's too much for
Coverity to track.

Adding the annotation ensures that any new callers to register_cfunc
don't pop up as new "leaks" in the Coverity scans.

src/nvim/eval/userfunc.c

@@ -3598,5 +3598,6 @@ char_u *register_cfunc(cfunc_T cb, cfunc_free_T cb_free, void *state)
   STRCPY(fp->uf_name, name);
   hash_add(&func_hashtab, UF2HIKEY(fp));
 
+  // coverity[leaked_storage]
   return fp->uf_name;
 }