Java: Minor perf fix.

Author: aschackmull

java/ql/lib/semmle/code/java/frameworks/Thrift.qll

@@ -8,6 +8,7 @@ import java
  * A file detected as generated by the Apache Thrift Compiler.
  */
 class ThriftGeneratedFile extends GeneratedFile {
+  cached
   ThriftGeneratedFile() {
     exists(JavadocElement t | t.getFile() = this |
       exists(string msg | msg = t.getText() | msg.regexpMatch("(?i).*\\bAutogenerated by Thrift.*"))