kvm: selftests: add -std=gnu99 cflags

lib/kvm_util.c: In function ‘kvm_memcmp_hva_gva’:
lib/kvm_util.c:332:2: error: ‘for’ loop initial declarations are only allowed in C99 mode

So add -std=gnu99 to CFLAGS

Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>