Add back include_gcs build constraint

Signed-off-by: Huu Nguyen <whoshuu@gmail.com>

registry/storage/driver/gcs/gcs.go

@@ -9,6 +9,8 @@
 //
 // Note that the contents of incomplete uploads are not accessible even though
 // Stat returns their length
+//
+// +build include_gcs
 
 package gcs