AP_Volz_Protocol: Moved member variable definition within proper #define block to prevent unused variable warning

libraries/AP_Volz_Protocol/AP_Volz_Protocol.h

@@ -88,7 +88,6 @@ private:
 
     // Loop in thread to output to uart
     void loop();
-    uint8_t sent_count;
 
     void init(void);
 
@@ -120,6 +119,8 @@ private:
     void read_telem();
     void process_response(const CMD &cmd);
 
+    uint8_t sent_count;
+
     struct {
         CMD_ID types[3] {
             CMD_ID::READ_CURRENT,