mk: fix build for clang 4

mk: fix build for clang 4

clang 4 gives "taking address of packed member may result in an
unaligned pointer value" warnings in a few locations [1].

Disabled "-Waddress-of-packed-member" warning for clang >= 4

[1] build errors:
.../lib/librte_eal/common/eal_common_memzone.c:275:25:
error: taking address of packed member 'mlock' of class or structure
'rte_mem_config' may result in an unaligned pointer value
[-Werror,-Waddress-of-packed-member]
        rte_rwlock_write_lock(&mcfg->mlock);
                               ^~~~~~~~~~~

.../lib/librte_ip_frag/rte_ipv4_reassembly.c:139:31:
error: taking address of packed member 'src_addr' of class or structure
'ipv4_hdr' may result in an unaligned pointer value
[-Werror,-Waddress-of-packed-member]
        psd = (unaligned_uint64_t *)&ip_hdr->src_addr;
                                     ^~~~~~~~~~~~~~~~

.../lib/librte_vhost/vhost_user.c:1037:34:
error: taking address of packed member 'payload' of class or structure
'VhostUserMsg' may result in an unaligned pointer value
[-Werror,-Waddress-of-packed-member]
	vhost_user_set_vring_num(dev, &msg.payload.state);
				       ^~~~~~~~~~~~~~~~~

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>