target/s390x: Fix brace Werror with clang 6.0.0

The warning is

target/s390x/misc_helper.c:209:21: error: suggest
      braces around initialization of subobject [-Werror,-Wmissing-braces]
    SysIB sysib = { 0 };
                    ^
                    {}

While the original code is correct, and technically exactly correct
as per ISO C89, both GCC and Clang support plain empty set of braces
as an extension.

Cc: Alexander Graf <agraf@suse.de>
Cc: David Hildenbrand <david@redhat.com>
Cc: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20180512045950.12386-5-richard.henderson@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

target/s390x/misc_helper.c

@@ -206,7 +206,7 @@ uint32_t HELPER(stsi)(CPUS390XState *env, uint64_t a0, uint64_t r0, uint64_t r1)
     const MachineState *ms = MACHINE(qdev_get_machine());
     uint16_t total_cpus = 0, conf_cpus = 0, reserved_cpus = 0;
     S390CPU *cpu = s390_env_get_cpu(env);
-    SysIB sysib = { 0 };
+    SysIB sysib = { };
     int i, cc = 0;
 
     if ((r0 & STSI_R0_FC_MASK) > STSI_R0_FC_LEVEL_3) {