This fixes GCC 7 compilation when GRAPHITE is selected.
The path is replaced with $(TOOLCHAIN_DIR), while it should be
$(STAGING_DIR_HOST). To keep in sync with the path of gmp,
mpfr and mpc, I'm using $(TOPDIR)/staging_dir/host.
Fixes: f62f4b3c5c ("build: stop overriding STAGING_DIR_HOST for toolchain build")
Signed-off-by: Syrone Wong <wong.syrone@gmail.com>