samples: bpf: include bpf/bpf.h instead of local libbpf.h

There are two files in the tree called libbpf.h which is becoming
problematic.  Most samples don't actually need the local libbpf.h
they simply include it to get to bpf/bpf.h.  Include bpf/bpf.h
directly instead.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

samples/bpf/syscall_tp_user.c

@@ -16,7 +16,7 @@
 #include <assert.h>
 #include <stdbool.h>
 #include <sys/resource.h>
-#include "libbpf.h"
+#include <bpf/bpf.h>
 #include "bpf_load.h"
 
 /* This program verifies bpf attachment to tracepoint sys_enter_* and sys_exit_*.