build: Fix 'make check' when libdpkg has not been built yet

Force the subdirectory build order, so that lib/dpkg gets built before
lib/dpkg/test, making the former always available for the latter.

lib/dpkg/Makefile.am

@@ -1,6 +1,6 @@
 ## Process this file with automake to produce Makefile.in
 
-SUBDIRS = test
+SUBDIRS = . test
 
 localedir = $(datadir)/locale
 pkgconfdir = $(sysconfdir)/@PACKAGE@