list: fix list handling for gcc10 (#383)

-Wpointer-arith has been removed from warnings as with GCC10 that warns every time qb_list_entry is used, which is just noise.

configure.ac

@@ -601,7 +601,6 @@ WARNLIST="
 	suggest-attribute=format
 	property-attribute-mismatch
 	strict-prototypes
-	pointer-arith
 	write-strings
 	cast-align
 	bad-function-cast

include/qb/qblist.h

@@ -193,8 +193,8 @@ static inline void qb_list_splice_tail(struct qb_list_head *list,
  * @param type:	the type of the struct this is embedded in.
  * @param member:	the name of the list_struct within the struct.
  */
-#define qb_list_entry(ptr,type,member)\
-	((type *)((char *)(ptr)-(char*)(&((type *)0)->member)))
+#define qb_list_entry(ptr,type,member) ({	       \
+	((type *)((void*)ptr - offsetof(type, member))); })
 
 /**
  * Get the first element from a list