Fix -fPIC linking error introduced in 6701817df9

Collectd package on fails to build when collectd-mod-netlink is enabled:
  [...]/usr/lib/libnetlink.a(libnetlink.o): relocation R_MIPS_26 against
  `close' can not be used when making a shared object; recompile with
  -fPIC

I believe this is caused by MAKE_FLAGS change introduced in
6701817df9a68bd79a88058ef5600b9c4f45a59e.

Anyway, reverting the iproute2's Makefile line with
  FPIC=""
back to
  FPIC="$(FPIC)"
seems to have solved the problem.

OpenWRT bug report: https://dev.openwrt.org/ticket/14642