clang-tidy: Fix override errors

The following errors were reported during the clang-tidy enablement
build due to overridden functions not being marked with the override
keyword. The proposed fix is to add the override keyword where it
was missing.

'''
inc/serial_uart_mux.hpp:74:10: error: 'handleEvent' overrides a member function but is not marked 'override' [-Werror
'''

Tested: Verified Build and unit testing.

Change-Id: I999968f538650fb4eefd1702fffcbe49d97d5747
Signed-off-by: Jayanth Othayoth <ojayanth@gmail.com>
diff --git a/inc/serial_uart_mux.hpp b/inc/serial_uart_mux.hpp
index 1e954da..57b4c58 100644
--- a/inc/serial_uart_mux.hpp
+++ b/inc/serial_uart_mux.hpp
@@ -70,7 +70,7 @@
     void configSerialConsoleMux(size_t position);
     bool isOCPDebugCardPresent();
 
-    void handleEvent(sd_event_source*, int, uint32_t) {}
+    void handleEvent(sd_event_source*, int, uint32_t) override {}
 
   protected:
     size_t gpioLineCount;