Move missing import into feature patches to fix pre-features compile

Came across this in my complete utter failure to use the commands to
apply patches properly

paper-server/patches/sources/io/papermc/paper/FeatureHooks.java.patch

@@ -1,6 +1,6 @@
 --- /dev/null
 +++ b/io/papermc/paper/FeatureHooks.java
-@@ -1,0 +_,233 @@
+@@ -1,0 +_,234 @@
 +package io.papermc.paper;
 +
 +import io.papermc.paper.command.PaperSubcommand;
@@ -10,6 +10,7 @@
 +import it.unimi.dsi.fastutil.objects.ObjectOpenHashSet;
 +import it.unimi.dsi.fastutil.objects.ObjectSet;
 +import it.unimi.dsi.fastutil.objects.ObjectSets;
++import java.util.HashMap;
 +import java.util.List;
 +import java.util.Map;
 +import java.util.Set;