uci: silence UBSAN error by using offsetof macro from compiler

Fixes following ubdefined-behavior as reported by clang version 10.0.0-4ubuntu1~18.04.2:

 delta.c:139:52: runtime error: member access within null pointer of type 'struct uci_element'
 SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior delta.c:139:52

Signed-off-by: Petr Štetiar <ynezz@true.cz>

uci.h

@@ -34,6 +34,7 @@ extern "C" {
 #include <setjmp.h>
 #include <stdio.h>
 #include <stdint.h>
+#include <stddef.h>
 
 #define UCI_CONFDIR "/etc/config"
 #define UCI_SAVEDIR "/tmp/.uci"
@@ -509,11 +510,6 @@ struct uci_parse_option {
 };
 
 
-/* linked list handling */
-#ifndef offsetof
-#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
-#endif
-
 /**
  * container_of - cast a member of a structure out to the containing structure
  * @ptr:    the pointer to the member.