Add required modules to module-info.java

[koppor]

Needed to add two require statements to our module-info.java to get JabRef compiling again.

Quick fix, thus, I merge instantly.

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[koppor]

Can't merge, because of following output:

Error: jdk.internal.org.objectweb.asm.MethodTooLargeException: Method too large: jdk/internal/module/SystemModules$all.moduleDescriptors ()[Ljava/lang/module/ModuleDescriptor;

Blocked by https://bugs.openjdk.java.net/browse/JDK-8240567.