Add lcd_buzz function to ultralcd.h so that non-LCD code can make use of lcd's buzzer.

Argument order of lcd_buzz was set to mirror that on tone(). Change
default M300 frequency to something audible.

Marlin/ultralcd.cpp

@@ -946,6 +946,13 @@ void lcd_buttons_update()
     lastEncoderBits = enc;
 }
 
+void lcd_buzz(long duration, uint16_t freq)
+{ 
+#ifdef LCD_USE_I2C_BUZZER
+  lcd.buzz(duration,freq);
+#endif   
+}
+
 bool lcd_clicked() 
 { 
   return LCD_CLICKED;