container_of: avoid using __extension__

We can just write the macro on one line.
diff --git a/src/util.h b/src/util.h
index 07654ec..49a1cd9 100644
--- a/src/util.h
+++ b/src/util.h
@@ -57,11 +57,8 @@
 #endif
 
 #ifndef container_of
-#define container_of(ptr, type, member)              \
-    __extension__({                                  \
-        void *__mptr = (void *)(ptr);                \
-        ((type *)(__mptr - offsetof(type, member))); \
-    })
+#define container_of(ptr, type, member) \
+        ((type *) (((char *)(ptr)) - offsetof(type, member)))
 #endif
 
 #ifndef G_SIZEOF_MEMBER