package/petitboot: fix install rule for po files

We need DESTDIR, so that we install to the target filesystem.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/openpower/package/petitboot/petitboot.mk b/openpower/package/petitboot/petitboot.mk
index c586874..1a5a0ab 100644
--- a/openpower/package/petitboot/petitboot.mk
+++ b/openpower/package/petitboot/petitboot.mk
@@ -56,7 +56,7 @@
 	ln -sf /usr/sbin/pb-udhcpc \
 		$(TARGET_DIR)/usr/share/udhcpc/default.script.d/
 
-	$(MAKE) -C $(@D)/po install
+	$(MAKE) -C $(@D)/po DESTDIR=$(TARGET_DIR) install
 endef
 
 PETITBOOT_POST_INSTALL_TARGET_HOOKS += PETITBOOT_POST_INSTALL