debian/rules.real: set absolute bpftool path for linux 6.8+
I rebased the Debian linux packaging onto 6.8-rc1 because the rk3588 patch stack uses that as a base. When building it, I ran into the following error which the commit in this MR fixes.
env -u ABINAME -u ARCH -u FEATURESET -u FLAVOUR -u VERSION -u LOCALVERSION DISTRIBUTION_OFFICIAL_BUILD=1 DISTRIBUTOR="Debian" DISTRIBUTION_VERSION="6.8~rc1-1" KBUILD_BUILD_TIMESTAMP="@1709407126" KBUILD_BUILD_VERSION_TIMESTAMP="Debian 6.8~rc1-1 (2024-03-02)" KBUILD_BUILD_USER="debian-kernel" KBUILD_BUILD_HOST="lists.debian.org" KBUILD_VERBOSE=1 /usr/bin/make KCFLAGS=-fdebug-prefix-map=/<<PKGBUILDDIR>>/= KBUILD_HOSTCFLAGS='' HOSTCFLAGS='' KBUILD_HOSTLDFLAGS='' -C 'debian/build/source_none/tools/bpf/bpftool' O=debian/build/build_arm64_none_arm64/tools/bpf/bpftool CROSS_COMPILE= FEATURE_TESTS= FEATURE_DISPLAY=
make[3]: Entering directory '/<<PKGBUILDDIR>>/tools/bpf/bpftool'
MKDIR /<<PKGBUILDDIR>>/debian/build/build_arm64_none_arm64/tools/bpf/bpftool/libbpf/
MKDIR /<<PKGBUILDDIR>>/debian/build/build_arm64_none_arm64/tools/bpf/bpftool/libbpf/include/bpf
INSTALL /<<PKGBUILDDIR>>/debian/build/build_arm64_none_arm64/tools/bpf/bpftool/libbpf/include/bpf/hashmap.h
INSTALL /<<PKGBUILDDIR>>/debian/build/build_arm64_none_arm64/tools/bpf/bpftool/libbpf/include/bpf/nlattr.h
make[4]: Entering directory '/<<PKGBUILDDIR>>/tools/lib/bpf'
INSTALL /<<PKGBUILDDIR>>/debian/build/build_arm64_none_arm64/tools/bpf/bpftool/libbpf/include/bpf/relo_core.h
INSTALL /<<PKGBUILDDIR>>/debian/build/build_arm64_none_arm64/tools/bpf/bpftool/libbpf/include/bpf/libbpf_internal.h
/<<PKGBUILDDIR>>/tools/scripts/Makefile.include:4: *** O=debian/build/build_arm64_none_arm64/tools/bpf/bpftool does not
exist. Stop.