Build issue on gcc-7

[vankoven]

Cant build Tempesta on gcc-7:

tempesta/tempesta_fw/http_limits.c: In function ‘frang_http_req_process’:
tempesta/tempesta_fw/http_limits.c:255:9: error: the omitted middle operand in ?: will always be ‘true’, suggest explicit middle operand [-Werror=parentheses]
       ? : tfw_vhost_global_frang_cfg()->member)  \
         ^
tempesta/tempesta_fw/http_limits.c:947:3: note: in expansion of macro ‘__FRANG_CFG_VAR’
   __FRANG_CFG_VAR(host_required, http_host_required);
   ^~~~~~~~~~~~~~~
tempesta/tempesta_fw/http_limits.c:255:9: error: the omitted middle operand in ?: will always be ‘true’, suggest explicit middle operand [-Werror=parentheses]
       ? : tfw_vhost_global_frang_cfg()->member)  \
         ^
tempesta/tempesta_fw/http_limits.c:958:3: note: in expansion of macro ‘__FRANG_CFG_VAR’
   __FRANG_CFG_VAR(ct_required, http_ct_required);
   ^~~~~~~~~~~~~~~

GCC 7 is not default on Debian 9, so low priority.