libbpf: Fix debian kernel version information parsing

Currently get_debian_kernel_version() expects the version to be in the
form of x.y.z but the Debian kernels uploaded to experimental are not
always LTS kernels. Like the current Debian kernel in experimental
is 7.2~rc7-1~exp1 and uname -v reported "Debian 7.2~rc7-1~exp1".

Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260831134537.1227914-1-sudipm.mukherjee@gmail.com