package/lz4: only use PREFIX for host installation

As noted by Peter Korsgaard, using DESTDIR=$(HOST_DIR) and PREFIX=/usr
for the host installation is useless and actually incorrect (the
pkg-config file contains incorrect paths). Instead, using
PREFIX=$(HOST_DIR)/usr is simpler, and actually correct.

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