samples/bpf: Fix build for task_fd_query_user.c

Add missing include for <linux/perf_event.h> which was removed from
perf-sys.h in commit 91854f9a ("perf tools: Move everything
related to sys_perf_event_open() to perf-sys.h").

Fixes: 91854f9a

 ("perf tools: Move everything related to sys_perf_event_open() to perf-sys.h")
Reported-by: KP Singh <kpsingh@google.com>
Reported-by: Florent Revest <revest@google.com>
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: KP Singh <kpsingh@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20191001112249.27341-1-bjorn.topel@gmail.com