s390/perf: fix compile error (undefined reference sie_exit)

The perf_event code references sie_exit even if KVM is not available.
So add proper ifdefs to fix this one:

arch/s390/built-in.o: In function `sys_call_table_emu':
(.rodata+0x2b98): undefined reference to `sie_exit'
arch/s390/built-in.o: In function `sys_call_table_emu':
(.rodata+0x2ba0): undefined reference to `sie_exit'
make: *** [vmlinux] Error 1

Reported-by: Zhouping Liu <zliu@redhat.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>