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: NKP Singh <kpsingh@google.com>
Reported-by: NFlorent Revest <revest@google.com>
Signed-off-by: NBjörn Töpel <bjorn.topel@intel.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Tested-by: NKP Singh <kpsingh@google.com>
Acked-by: NSong Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20191001112249.27341-1-bjorn.topel@gmail.com

samples/bpf/task_fd_query_user.c

@@ -13,6 +13,7 @@
 #include <sys/resource.h>
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <linux/perf_event.h>
 
 #include "libbpf.h"
 #include "bpf_load.h"