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.h

@@ -34,6 +34,8 @@
   extern int absPreheatHPBTemp;
   extern int absPreheatFanSpeed;
     
+  void lcd_buzz(long duration,uint16_t freq);
+
   bool lcd_clicked();
 
 #else //no lcd
@@ -42,6 +44,7 @@
   FORCE_INLINE void lcd_setstatus(const char* message) {}
   FORCE_INLINE void lcd_buttons_update() {}
   FORCE_INLINE void lcd_reset_alert_level() {}
+  FORCE_INLINE void lcd_buzz(long duration,uint16_t freq) {}
 
   #define LCD_MESSAGEPGM(x) 
   #define LCD_ALERTMESSAGEPGM(x)