Revert lcd_putchar to type int and retrun 0

Discussion: https://github.com/prusa3d/Prusa-Firmware/pull/2008#issuecomment-514500738

Firmware/lcd.cpp

@@ -189,9 +189,10 @@ static void lcd_begin(uint8_t clear)
 	#endif
 }
 
-static void lcd_putchar(char c, FILE *)
+static int lcd_putchar(char c, FILE *)
 {
 	lcd_write(c);
+	return 0;
 }
 
 void lcd_init(void)