s390/ap: explicitly include ultravisor header

The ap_bus is using inline functions of the ultravisor (uv) in-kernel
API. The related header file is implicitly included via several other
headers. Replace this by an explicit include of the ultravisor header
in the ap_bus file.

Signed-off-by: Holger Dengler <dengler@linux.ibm.com>
Reviewed-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>