Fixed compilation when using avr-gcc

- avr-gcc is more strict than the Arduino IDE, it seems.

Marlin/src/lcd/dogm/ultralcd_impl_st7920_lite_status_screen.h

     static void draw_fan_speed(uint8_t value);
     static void draw_print_time(uint32_t elapsed);
     static void draw_feedrate_percentage(uint8_t percentage);
-    static void draw_status_message(const unsigned char *str);
+    static void draw_status_message(const char *str);
     static void draw_position(const float x, const float y, const float z, bool position_known = true);
 
     static bool indicators_changed();

Marlin/src/lcd/dogm/ultralcd_impl_st7920_lite_status_screen_impl.h

   write_str_P((const char*)str);
 }
 
-void ST7920_Lite_Status_Screen::write_number(uint8_t value, uint8_t digits=3) {
+void ST7920_Lite_Status_Screen::write_number(uint8_t value, uint8_t digits) {
   char  str[7];
   const char *fmt;
   switch(digits) {
   #endif
 }
 
-void ST7920_Lite_Status_Screen::draw_status_message(const unsigned char *str) {
+void ST7920_Lite_Status_Screen::draw_status_message(const char *str) {
   set_ddram_address(DDRAM_LINE_4);
   begin_data();
   #if ENABLED(STATUS_MESSAGE_SCROLLING)