perf x86 kvm-stat: Add missing string.h header

It uses strcmp(), strstr() and was getting the required string.h header
by luck, from evsel.h -> cpumap.h -> debug.h -> string.h, add the
missing header.

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-qrz8hhvrhwnmt5ocfwk4br5d@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/arch/x86/util/kvm-stat.c

 // SPDX-License-Identifier: GPL-2.0
 #include <errno.h>
+#include <string.h>
 #include "../../../util/kvm-stat.h"
 #include "../../../util/evsel.h"
 #include <asm/svm.h>