s390x: Avoid variable size warning in ipl.h

s390x: Avoid variable size warning in ipl.h

S390IPLState contains two IplParameterBlock, which may in turn have
either a IPLBlockPV or a IplBlockFcp, both ending with a variable
sized field (an array).

This causes a warning with clang 11 or greater, which checks that
variable sized type are only allocated at the end of the struct:

In file included from ../qemu-cfi-v3/target/s390x/diag.c:21:
../qemu-cfi-v3/hw/s390x/ipl.h:161:23: error: field 'iplb' with variable sized type 'IplParameterBlock' (aka 'union IplParameterBlock') not at the end of a struct or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
    IplParameterBlock iplb;
                      ^
../qemu-cfi-v3/hw/s390x/ipl.h:162:23: error: field 'iplb_pv' with variable sized type 'IplParameterBlock' (aka 'union IplParameterBlock') not at the end of a struct or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
    IplParameterBlock iplb_pv;

In this case, however, the warning is a false positive, because
IPLBlockPV and IplBlockFcp are allocated in a union wrapped at 4K,
making the union non-variable sized.

Fix the warning by turning the two variable sized arrays into arrays
of size 0. This avoids the compiler error and should produce the
same code.

Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
Message-Id: <20201105221905.1350-5-dbuono@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>