Fix kconfig warnings (#1967)

* quote string defaults for Kconfig values to eliminate warnings

* don't add \ in default for LV_TICK_CUSTOM_SYS_TIME_EXPR
(tho its needed on linux and MacOS) as I can't test on Windows

Kconfig

@@ -112,19 +112,19 @@ menu "LVGL configuration"
     config LV_MEM_CUSTOM_INCLUDE
         string
         prompt "Header to include for the custom memory function"
-        default stdlib.h
+        default "stdlib.h"
         depends on LV_MEM_CUSTOM
 
     config LV_MEM_CUSTOM_ALLOC
         string
         prompt "Wrapper to malloc"
-        default malloc 
+        default "malloc" 
         depends on LV_MEM_CUSTOM
 
     config LV_MEM_CUSTOM_FREE
         string
         prompt "Wrapper to free"
-        default free 
+        default "free" 
         depends on LV_MEM_CUSTOM
 
 	config LV_MEM_SIZE_BYTES
@@ -286,13 +286,13 @@ menu "LVGL configuration"
         config LV_TICK_CUSTOM_INCLUDE
             string
             prompt "Header for the system time function"
-            default Arduino.h
+            default "Arduino.h"
             depends on LV_TICK_CUSTOM
 
         config LV_TICK_CUSTOM_SYS_TIME_EXPR
             string
             prompt "Expression evaluating to current system time in ms"
-            default "(millis())"
+            default "millis()"
             depends on LV_TICK_CUSTOM
     endmenu