perf kvm s390: Add missing string.h header

It uses strstr(), needs to include string.h or its not going to build
when we remove string.h from the place it is getting from indirectly, by
luck.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-72y0i0uiaqght5b83e3ae7p4@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>