python: add correct host expat

Signed-off-by: Steven Barth <steven@midlink.org>

lang/python/Makefile

@@ -221,6 +221,7 @@ HOST_CONFIGURE_ARGS+= \
 	--without-cxx-main \
 	--without-pymalloc \
 	--with-threads \
+	--with-system-expat=$(STAGING_DIR_HOST) \
 	--prefix=$(STAGING_DIR_HOST) \
 	--with-ensurepip=upgrade \
 	CONFIG_SITE= \