AP_OpticalFlow: correct compiler warning

In file included from
../../libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.cpp:28:
../../libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.h:75:13: warning:
private field 'sum_x' is not used [-Wunused-private-field]
    int32_t sum_x;
            ^
../../libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.h:76:13: warning:
private field 'sum_y' is not used [-Wunused-private-field]
    int32_t sum_y;
            ^
../../libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.h:77:14: warning:
private field 'last_print_ms' is not used [-Wunused-private-field]
    uint32_t last_print_ms;
             ^
4 warnings generated.

libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.cpp

@@ -309,11 +309,14 @@ void AP_OpticalFlow_Pixart::timer(void)
     }
     
 #if 0
+    static uint32_t last_print_ms;
     static int fd = -1;
     if (fd == -1) {
         fd = open("/dev/ttyACM0", O_WRONLY);
     }
     // used for debugging
+    static int32_t sum_x;
+    static int32_t sum_y;
     sum_x += burst.delta_x;
     sum_y += burst.delta_y;
     

libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.h

@@ -72,9 +72,6 @@ private:
     void timer(void);
     void motion_burst(void);
 
-    int32_t sum_x;
-    int32_t sum_y;
-    uint32_t last_print_ms;
     uint32_t last_burst_us;
     uint32_t last_update_ms;
 };