target/s390x: Fix brace Werror with clang 6.0.0

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 <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Cornelia Huck <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
Message-Id: <20180512045950[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>