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

samples/bpf/Makefile

@@ -197,7 +197,7 @@ $(obj)/tracex5_kern.o: $(obj)/syscall_nrs.h
 # 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 \