[S390] add types.h include to s390_ext.h

The header file for external interrupts uses the _u16 type. Make sure
that _u16 is defined by including linux/types.h. This prevents compile
failures, if asm/s390_ext.h is the first include file.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>