Removed NXT_PACKED from port structures.

That was added only to silence Valgrind, but it results in
unaligned access (which breaks Unit on ARM architecture).

src/nxt_port.h

@@ -32,7 +32,7 @@ typedef struct {
 
     /* Message data send using mmap, next chunk is a nxt_port_mmap_msg_t. */
     uint8_t              mmap;      /* 1 bit */
-} NXT_PACKED nxt_port_msg_t;
+} nxt_port_msg_t;
 
 
 typedef struct {
@@ -99,7 +99,7 @@ typedef struct {
     size_t              max_size;
     size_t              max_share;
     nxt_process_type_t  type:8;
-} NXT_PACKED nxt_port_msg_new_port_t;
+} nxt_port_msg_new_port_t;
 
 
 /*