kvmtool: Fix absence of __packed definition

The absence of __packed definition in kvm/compiler.h cause build
failer after syncing kernel headers with Linux-6.8 because the
kernel header uapi/linux/virtio_pci.h uses __packed for structures.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20240325153141.6816-3-apatel@ventanamicro.com


Signed-off-by: Will Deacon <will@kernel.org>