samples/bpf: Fix broken vmlinux path for VMLINUX_BTF

Commit 13b25489 ("kbuild: change working directory to external
module directory with M=") changed kbuild working directory of bpf
sample programs to samples/bpf, which broke the vmlinux path for
VMLINUX_BTF, as the Makefiles assume the current work directory to be
the kernel output directory and use a relative path (i.e., ./vmlinux):

  Makefile:316: *** Cannot find a vmlinux for VMLINUX_BTF at any of "  /path/to/linux/samples/bpf/vmlinux", build the kernel or set VMLINUX_BTF like "VMLINUX_BTF=/sys/kernel/btf/vmlinux" or VMLINUX_H variable.  Stop.

Correctly refer to the kernel output directory using $(objtree).

Fixes: 13b25489 ("kbuild: change working directory to external module directory with M=")
Signed-off-by: Jinghao Jia <jinghao7@illinois.edu>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Ruowen Qin <ruqin@redhat.com>
Link: https://lore.kernel.org/bpf/20250203085506.220297-3-jinghao7@illinois.edu