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>
 # But, there is no easy way to fix it, so just exclude it since it is
 # useless for BPF samples.
 $(obj)/%.o: $(src)/%.c
-       $(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) \
+       $(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) -I$(obj) \
                -D__KERNEL__ -D__ASM_SYSREG_H -Wno-unused-value -Wno-pointer-sign \
                -Wno-compare-distinct-pointer-types \
                -Wno-gnu-variable-sized-type-not-at-end \