node_states: Mark relevant member functions as const

src/audio_core/renderer/nodes/node_states.h

@@ -56,7 +56,7 @@ class NodeStates {
          *
          * @return The current stack position.
          */
-        u32 Count() {
+        u32 Count() const {
             return pos;
         }
 
@@ -83,7 +83,7 @@ class NodeStates {
          *
          * @return The node on the top of the stack.
          */
-        u32 top() {
+        u32 top() const {
             return stack[pos - 1];
         }