macro: add macro for determining size of struct with trailing union

src/basic/macro.h

@@ -248,6 +248,7 @@ static inline int __coverity_check_and_return__(int condition) {
 #define char_array_0(x) x[sizeof(x)-1] = 0;
 
 #define sizeof_field(struct_type, member) sizeof(((struct_type *) 0)->member)
+#define endoffsetof_field(struct_type, member) (offsetof(struct_type, member) + sizeof_field(struct_type, member))
 
 /* Returns the number of chars needed to format variables of the specified type as a decimal string. Adds in
  * extra space for a negative '-' prefix for signed types. Includes space for the trailing NUL. */