fix build

Author: GoeLin

src/java.base/share/classes/java/lang/AbstractStringBuilder.java

@@ -28,6 +28,7 @@
 import jdk.internal.math.DoubleToDecimal;
 import jdk.internal.math.FloatToDecimal;
 
+import java.io.IOException;
 import java.nio.CharBuffer;
 import java.util.Arrays;
 import java.util.Spliterator;