libhwc2.1: use const for getTargetOperationRate()

Bug: 296163848
Test: build pass
Change-Id: Ia74ee6a070f261216b90a8b12900f3becc1d7409
diff --git a/libhwc2.1/libdevice/ExynosDisplay.h b/libhwc2.1/libdevice/ExynosDisplay.h
index 8bdaad5..f744cc1 100644
--- a/libhwc2.1/libdevice/ExynosDisplay.h
+++ b/libhwc2.1/libdevice/ExynosDisplay.h
@@ -1663,7 +1663,7 @@
             virtual int32_t onConfig(hwc2_config_t __unused cfg) { return 0; }
             virtual int32_t onBrightness(uint32_t __unused dbv) { return 0; }
             virtual int32_t onPowerMode(int32_t __unused mode) { return 0; }
-            virtual int32_t getTargetOperationRate() { return 0; }
+            virtual int32_t getTargetOperationRate() const { return 0; }
         };
 
     public: