tcp_priv.h: LWIP_TCP_OPT_LENGTH: Enclosing macro argument in parentheses

Fix below build error:
In file included from
../../../../lwip/src/../test/unit/tcp/test_tcp.c:3:0:
../../../../lwip/src/../test/unit/tcp/test_tcp.c: In function
‘test_tcp_rto_timeout_syn_sent_impl’:
../../../../lwip/src/../test/unit/tcp/test_tcp.c:1246:113: error: suggest parentheses around arithmetic in operand of ‘|’ [-Werror=parentheses]
   const u16_t tcp_syn_opts_len = LWIP_TCP_OPT_LENGTH(TF_SEG_OPTS_MSS|TF_SEG_OPTS_WND_SCALE|TF_SEG_OPTS_SACK_PERM|TF_SEG_OPTS_TS);
                                                                                                                 ^
../../../../lwip/src/include/lwip/priv/tcp_priv.h:305:4: note: in definition of macro ‘LWIP_TCP_OPT_LENGTH’
   (flags & TF_SEG_OPTS_MSS       ? LWIP_TCP_OPT_LEN_MSS           : 0) + \
    ^~~~~

Signed-off-by: Axel Lin <axel.lin@ingics.com>

src/include/lwip/priv/tcp_priv.h

@@ -302,10 +302,10 @@ struct tcp_seg {
 #endif
 
 #define LWIP_TCP_OPT_LENGTH(flags) \
-  (flags & TF_SEG_OPTS_MSS       ? LWIP_TCP_OPT_LEN_MSS           : 0) + \
-  (flags & TF_SEG_OPTS_TS        ? LWIP_TCP_OPT_LEN_TS_OUT        : 0) + \
-  (flags & TF_SEG_OPTS_WND_SCALE ? LWIP_TCP_OPT_LEN_WS_OUT        : 0) + \
-  (flags & TF_SEG_OPTS_SACK_PERM ? LWIP_TCP_OPT_LEN_SACK_PERM_OUT : 0)
+  ((flags) & TF_SEG_OPTS_MSS       ? LWIP_TCP_OPT_LEN_MSS           : 0) + \
+  ((flags) & TF_SEG_OPTS_TS        ? LWIP_TCP_OPT_LEN_TS_OUT        : 0) + \
+  ((flags) & TF_SEG_OPTS_WND_SCALE ? LWIP_TCP_OPT_LEN_WS_OUT        : 0) + \
+  ((flags) & TF_SEG_OPTS_SACK_PERM ? LWIP_TCP_OPT_LEN_SACK_PERM_OUT : 0)
 
 /** This returns a TCP header option for MSS in an u32_t */
 #define TCP_BUILD_MSS_OPTION(mss) lwip_htonl(0x02040000 | ((mss) & 0xFFFF))