Add security tag to java/random-used-once

Raised in #7601, this is one of the only .ql files that has a security-severity score but not the tag "security", including many other queries that live outside the `Security/` subdirectory.

Besides this the only other files with this security-severity-but-no-security-tag combination are:

```
java/ql/src/Frameworks/JavaEE/EJB/EjbContainerInterference.ql
java/ql/src/Frameworks/JavaEE/EJB/EjbFileIO.ql
java/ql/src/Frameworks/JavaEE/EJB/EjbNative.ql
java/ql/src/Frameworks/JavaEE/EJB/EjbReflection.ql
java/ql/src/Frameworks/JavaEE/EJB/EjbSecurityConfiguration.ql
java/ql/src/Frameworks/JavaEE/EJB/EjbSerialization.ql
java/ql/src/Frameworks/JavaEE/EJB/EjbSetSocketOrUrlFactory.ql
```

Given their location I'm assuming these queries are disabled by default and likely shouldn't changed?

Author: smowton

java/ql/src/Likely Bugs/Arithmetic/RandomUsedOnce.ql

@@ -9,6 +9,7 @@
  * @id java/random-used-once
  * @tags reliability
  *       maintainability
+ *       security
  *       external/cwe/cwe-335
  */