perf tools riscv: Fix build error on riscv due to missing header for 'struct perf_sample'

Since the definition of 'struct perf_sample' has been moved to sample.h,
we need to include this header file to fix the build error as follows:

  arch/riscv/util/unwind-libdw.c: In function 'libdw__arch_set_initial_registers':
  arch/riscv/util/unwind-libdw.c:12:50: error: invalid use of undefined type 'struct perf_sample'
     12 |         struct regs_dump *user_regs = &ui->sample->user_regs;
        |                                                  ^~

Fixes: 9823147da6c893d9 ("perf tools: Move 'struct perf_sample' to a separate header file to disentangle headers")
Signed-off-by: Eric Lin <eric.lin@sifive.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: greentime.hu@sifive.com
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: linux-riscv@lists.infradead.org
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Vincent Chen <vincent.chen@sifive.com>
Link: https://lore.kernel.org/r/20221231052731.24908-1-eric.lin@sifive.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/arch/riscv/util/unwind-libdw.c

@@ -4,7 +4,7 @@
 #include <elfutils/libdwfl.h>
 #include "../../util/unwind-libdw.h"
 #include "../../util/perf_regs.h"
-#include "../../util/event.h"
+#include "../../util/sample.h"
 
 bool libdw__arch_set_initial_registers(Dwfl_Thread *thread, void *arg)
 {