samples/bpf: Fix build error due to -isystem removal

Since recent Kbuild updates we no longer include files from compiler
directories. However, samples/bpf/hbm_kern.h hasn't been tuned for
this (LLVM 13):

  CLANG-bpf  samples/bpf/hbm_out_kern.o
In file included from samples/bpf/hbm_out_kern.c:55:
samples/bpf/hbm_kern.h:12:10: fatal error: 'stddef.h' file not found
         ^~~~~~~~~~
1 error generated.
  CLANG-bpf  samples/bpf/hbm_edt_kern.o
In file included from samples/bpf/hbm_edt_kern.c:53:
samples/bpf/hbm_kern.h:12:10: fatal error: 'stddef.h' file not found
         ^~~~~~~~~~
1 error generated.

It is enough to just drop both stdbool.h and stddef.h from includes
to fix those.

Fixes: 04e85bbf71c9 ("isystem: delete global -isystem compile option")
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Link: https://lore.kernel.org/bpf/20211115130741.3584-1-alexandr.lobakin@intel.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

samples/bpf/hbm_kern.h

@@ -9,8 +9,6 @@
  * Include file for sample Host Bandwidth Manager (HBM) BPF programs
  */
 #define KBUILD_MODNAME "foo"
-#include <stddef.h>
-#include <stdbool.h>
 #include <uapi/linux/bpf.h>
 #include <uapi/linux/if_ether.h>
 #include <uapi/linux/if_packet.h>