perf build: Update to C standard to gnu11

C11 has become the standard for mainstream kernel development [1],
allowing it in the perf build enables libraries like stdatomic.h to be
assumed to be present. This came up in the context of [2].

[1] https://lore.kernel.org/lkml/CAHk-=whWbENRz-vLY6vpESDLj6kGUTKO3khGtVfipHqwewh2HQ@mail.gmail.com/
[2] https://lore.kernel.org/lkml/20221024011024.462518-1-irogers@google.com/Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Bayduraev <alexey.v.bayduraev@linux.intel.com>
Cc: German Gomez <german.gomez@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20221024181913.630986-2-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.config

@@ -307,7 +307,7 @@ CORE_CFLAGS += -ggdb3
 CORE_CFLAGS += -funwind-tables
 CORE_CFLAGS += -Wall
 CORE_CFLAGS += -Wextra
-CORE_CFLAGS += -std=gnu99
+CORE_CFLAGS += -std=gnu11
 
 CXXFLAGS += -std=gnu++14 -fno-exceptions -fno-rtti
 CXXFLAGS += -Wall