samples/bpf: fix a build problem

tracex5_kern.c build failed with the following error message:
  ../samples/bpf/tracex5_kern.c:12:10: fatal error: 'syscall_nrs.h' file not found
  #include "syscall_nrs.h"
The generated file syscall_nrs.h is put in build/samples/bpf directory,
but this directory is not in include path, hence build failed.

The fix is to add $(obj) into the clang compilation path.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>