Internals: Add missing const. No functional change.

src/V3Randomize.cpp

@@ -126,7 +126,7 @@ private:
 
     // STATE
     AstNodeModule* m_modp = nullptr;  // Current module
-    AstNodeFTask* m_ftaskp = nullptr;  // Current function/task
+    const AstNodeFTask* m_ftaskp = nullptr;  // Current function/task
     size_t m_enumValueTabCount = 0;  // Number of tables with enum values created
     int m_randCaseNum = 0;  // Randcase number within a module for var naming