pps: fix a missing include in Intel pps driver

In file included from ../src/intel/ds/intel_pps_perf.cc:8:
../src/intel/ds/intel_pps_perf.h:34:78: error: ‘string’ in namespace ‘std’ does not name a type

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10934>

src/intel/ds/intel_pps_perf.h

@@ -8,6 +8,7 @@
 #pragma once
 
 #include <optional>
+#include <string>
 #include <vector>
 
 #include <intel/dev/intel_device_info.h>