libqrtr: Use size_t for offsets

Currently, offsetof produces an unsigned long, while qmi_elem_info's
offset member is a uint32_t. Since this is not the same type on all
platforms, this change uses a size_t for both. stddef.h is also
included for the standard offsetof definition, which also produces a
size_t.

This change also makes similar modifications to list.h.

src/list.h

@@ -1,8 +1,10 @@
 #ifndef _LIST_H_
 #define _LIST_H_
 
+#include <stddef.h>
+
 #ifndef offsetof
-#define offsetof(type, md) ((unsigned long)&((type *)0)->md)
+#define offsetof(type, md) ((size_t)&((type *)0)->md)
 #endif
 
 #ifndef container_of