configure_hotkey: Make IsUsedKey() a const member function

This doesn't actually modify instance state of the dialog, so this can
be made const.

src/yuzu/configuration/configure_hotkeys.cpp

@@ -90,7 +90,7 @@ void ConfigureHotkeys::Configure(QModelIndex index) {
     }
 }
 
-bool ConfigureHotkeys::IsUsedKey(QKeySequence key_sequence) {
+bool ConfigureHotkeys::IsUsedKey(QKeySequence key_sequence) const {
     return GetUsedKeyList().contains(key_sequence);
 }