WSL: Only skip ".dll" files for *executable* completions

This was overzealous and didn't allow anything named ".dll" in any
file completions.

This allows us to now add the cd completion fast path for WSL

src/wildcard.cpp

@@ -474,7 +474,7 @@ static bool wildcard_test_flags_then_complete(const wcstring &filepath, const wc
         return false;
     }
 
-    if (is_windows_subsystem_for_linux() &&
+    if (executables_only && is_windows_subsystem_for_linux() &&
         string_suffixes_string_case_insensitive(L".dll", filename)) {
         return false;
     }