build: disable -Wmissing-braces on older compilers

GCC 4.9 and newer stopped warning for missing braces around the
"universal" C zero initializer {0}.  One such initializer sneaked
into scsi/qemu-pr-helper.c and is breaking the build with such
older GCC versions.

Detect the lack of support for the idiom, and disable the warning
in that case.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
