Ensure source code in framework-docs is compiled in the build

This also ensures that the source code can be properly imported into an
IDE.

Author: sbrannen

framework-docs/framework-docs.gradle

@@ -1,5 +1,6 @@
 description = "Spring Framework Docs"
 
+apply plugin: 'java'
 apply plugin: 'kotlin'
 apply plugin: 'org.asciidoctor.jvm.convert'
 apply plugin: 'org.asciidoctor.jvm.pdf'