libbpf: Refactor header installation portions of Makefile

As we gradually get more headers that have to be installed, it's quite
annoying to copy/paste long $(call) commands. So extract that logic and do
a simple $(foreach) over the list of headers.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210603004026.2698513-3-andrii@kernel.org