libbpf: Install skel_internal.h header used from light skeletons

Light skeleton code assumes skel_internal.h header to be installed system-wide
by libbpf package. Make sure it is actually installed.

Fixes: 67234743 ("libbpf: Generate loader program out of BPF ELF file.")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210603004026.2698513-4-andrii@kernel.org