linux/types.h: switch back to include <asm/types.h>

Commit f23171ca (kvmtool: remove reference to <asm/types.h> from
BIOS include files) switched kvmtool's linux/types.h to include
<asm-generic/types.h>, which is actually what most architecture's
asm/types.h is including only anyways. But PowerPC is an exception,
so this commit broke compilation on PowerPC.
It turns out that the commits following the above mentioned one
remove the need to include the linux/types.h header for the BIOS
binaries at all, allowing us to actually switch back to include the
standard asm/types.h header again.
This allows the PowerPC port of kvmtool to build again.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>