libbpf: Makefile set specified permission mode

The third parameter to do_install was not used by $(INSTALL) command.
Fix this by only setting the -m option when the third parameter is supplied.

The use of a third parameter was introduced in commit  eb54e522 ("bpf:
install libbpf headers on 'make install'").

Without this change, the header files are install as executables files (755).

Fixes: eb54e522 ("bpf: install libbpf headers on 'make install'")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>