Mark ToolExecBase#getProblems as internal from Gradle

Prevents Gradle from treating `#getProblems` as a property getter,
causing plugin validation issues.

Author: Jonathing

src/main/groovy/net/minecraftforge/gradleutils/shared/ToolExecBase.java

@@ -76,6 +76,7 @@ protected ToolExecBase(Class<P> problemsType, Tool tool) {
     /// The enhanced problems instance to use for this task.
     ///
     /// @return The enhanced problems
+    @Internal
     protected final P getProblems() {
         return this.problems;
     }