pciutils: fix build with external toolchains

Buildroot Makefile to compile pciutils wasn't passing TARGET_LDFLAGS
to pciutils build system. Therefore, at link time, the pciutils build
system was not taking into account the important --sysroot option,
which was breaking the link with external toolchains.

Fixes bug #523.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>