Polish contrib Makefile

Avoid this warning:

autoreconf2.69: running: automake --add-missing --copy --no-force
vtc.am:12: warning: 'TEST_EXTENSIONS' cannot have conditional contents
contrib/Makefile.am:12:   'vtc.am' included from here

Always defining the tests even if contrib is not built should not cause
any harm.

contrib/Makefile.am

 #
 
+include $(top_srcdir)/vtc.am
+
 if !WITH_CONTRIB
 dist_noinst_SCRIPTS = \
 	varnishstatdiff
@@ -9,7 +11,6 @@ dist_bin_SCRIPTS = \
 
 TESTS = @CONTRIB_TESTS@
 
-include $(top_srcdir)/vtc.am
 endif
 
 EXTRA_DIST = @CONTRIB_TESTS@