perf s390: Always build with -fPIC

On s390, object files must be compiled with position-indepedent code in
order to be incrementally linked or linked to shared libraries.

Therefore, add -fPIC to the CFLAGS for s390 to ensure each object file
is built properly.

Reported-by: Jonathan Hermann <jonathan.hermann@de.ibm.com>
Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Thomas Richter <tmricht@linux.vnet.ibm.com>
Cc: linux s390 list <linux-s390@vger.kernel.org>
Link: https://lkml.kernel.org/r/20171207080951.GC4889@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>