bpftools: fix libbpf pkgconfig file

bpftools: fix libbpf pkgconfig file

The pkgconfig file hardcodes a host library directory which cannot be
overridden by OpenWrt during builds. Use SED to fix this and potential
include directory problems, as is done with several other packages.

This fixes a strange issue intermittently seen building iproute2 on the
oxnas target:

iptables modules directory: /usr/lib/iptables
libc has setns: yes
SELinux support: no
libbpf support: no
	libbpf version 0.3.0 is too low, please update it to at least 0.1.0
	LIBBPF_FORCE=on set, but couldn't find a usable libbpf

Fixes: 2f0d672088c3 ("bpftools: add utility and library packages
supporting eBPF usage")
Reported-by: Russell Senior <russell@personaltelco.net>
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>