iser: Fix a compiler warning triggered by the container_of() definition

This patch fixes the following compiler warning:

iser.c:338:14: error: cast from 'uint8_t *' (aka 'unsigned char *') to 'struct iser_pdu *' increases required alignment from 1 to 8 [-Werror,-Wcast-align]
                iser_pdu = container_of(pdu, struct iser_pdu, iscsi_pdu);
                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

lib/iser.c

@@ -46,7 +46,7 @@
   *
  */
 #define container_of(ptr, type, member) \
-    ((type *) ((uint8_t *)(ptr) - offsetof(type, member)))
+        ((type *)(void *)((uint8_t *)(ptr) - offsetof(type, member)))
 #endif