Set correct subdir path for new GP contrib

Reviewed-by: NHeikki Linnakangas <hlinnakangas@pivotal.io>

gpcontrib/gp_sparse_vector/Makefile

@@ -16,7 +16,7 @@ ifdef USE_PGXS
 	PGXS := $(shell pg_config --pgxs)
 	include $(PGXS)
 else
-	subdir = contrib/gp_sparse_vector
+	subdir = gpcontrib/gp_sparse_vector
 	top_builddir = ../..
 	include $(top_builddir)/src/Makefile.global
 	include $(top_srcdir)/contrib/contrib-global.mk