samples/hid: fix broken vmlinux path for VMLINUX_BTF

Commit 13b25489b6f8 ("kbuild: change working directory to external
module directory with M=") changed kbuild working directory of hid-bpf
sample programs to samples/hid, 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:173: *** Cannot find a vmlinux for VMLINUX_BTF at any of " /path/to/linux/samples/hid/vmlinux", build the kernel or set VMLINUX_BTF or VMLINUX_H variable. Stop.

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

Fixes: 13b25489b6f8 ("kbuild: change working directory to external module directory with M=")
Tested-by: Ruowen Qin <ruqin@redhat.com>
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jinghao Jia <jinghao7@illinois.edu>
Link: https://patch.msgid.link/20250203085506.220297-4-jinghao7@illinois.edu
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>