Disable gnu-variable-sized-type-not-at-end so that tests and libraries
can create overlays of variable sized arrays at the end of structs when
using a fixed number of entries, e.g. to get/set a single MSR.

It's possible to fudge around the warning, e.g. by defining a custom
struct that hardcodes the number of entries, but that is a burden for
both developers and readers of the code.

lib/x86_64/processor.c:664:19: warning: field 'header' with variable sized type 'struct kvm_msrs'
not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
                struct kvm_msrs header;
                                ^
lib/x86_64/processor.c:772:19: warning: field 'header' with variable sized type 'struct kvm_msrs'
not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
                struct kvm_msrs header;
                                ^
lib/x86_64/processor.c:787:19: warning: field 'header' with variable sized type 'struct kvm_msrs'
not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
                struct kvm_msrs header;
                                ^
3 warnings generated.

x86_64/hyperv_tlb_flush.c:54:18: warning: field 'hv_vp_set' with variable sized type 'struct hv_vpset'
not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
        struct hv_vpset hv_vp_set;
                        ^
1 warning generated.

x86_64/xen_shinfo_test.c:137:25: warning: field 'info' with variable sized type 'struct kvm_irq_routing'
not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
        struct kvm_irq_routing info;
                               ^
1 warning generated.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221213001653.3852042-12-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>