s390/uv: export uv_pin_shared for direct usage

Export the uv_pin_shared function so that it can be called from other
modules that carry a GPL-compatible license.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Tested-by: Viktor Mihajlovski <mihajlov@linux.ibm.com>
Link: https://lore.kernel.org/r/20230815184333.6554-11-akrowiak@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>