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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

samples/bpf/Makefile

···
 # 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 \