Add JarUtil.getJarAttribute to proguard rules

This method is used by serialization plugin to read runtime JAR
manifest.

Maven serialization plugin seems to use proguarded compiler
distribution,
so to use this method, it should be kept.

Size increased for about 2kb.

prepare/compiler/compiler.pro

@@ -243,6 +243,12 @@
 
 -keep class com.intellij.openapi.vfs.impl.jar.CoreJarFileSystem { *; }
 
+# Serialization plugin
+
+-keep class com.intellij.openapi.util.io.JarUtil {
+    public static java.lang.String getJarAttribute(java.io.File, java.util.jar.Attributes$Name);
+}
+
 # used in REPL
 # TODO: pack jline directly to scripting-compiler jars instead
 -keep class org.jline.reader.LineReaderBuilder { *; }