[S390] make appldata compile w/o CONFIG_SWAP

Avoid compile error by using EXPORT_SYMBOL_GPL(si_swapinfo) only if
CONFIG_SWAP is set.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>