Add virtual keyword to virtual method override, for clarity

These are already treated as virtual by the compiler anyway. Ideally we'd have
the override keyword here too, but I'm not sure if the lcd compiler supports
this.

Author: eric-wieser

cores/arduino/UARTClass.h

@@ -47,12 +47,12 @@ class UARTClass : public HardwareSerial
     void begin(const uint32_t dwBaudRate);
     void begin(const uint32_t dwBaudRate, const UARTModes config);
     void end(void);
-    int available(void);
+    virtual int available(void);
     int availableForWrite(void);
-    int peek(void);
-    int read(void);
-    void flush(void);
-    size_t write(const uint8_t c);
+    virtual int peek(void);
+    virtual int read(void);
+    virtual void flush(void);
+    virtual size_t write(const uint8_t c);
     using Print::write; // pull in write(str) and write(buf, size) from Print
 
     void setInterruptPriority(uint32_t priority);