Oops

Commit 1c1945fd9dbaf217062596062f73beac4934d7b6 broke compilation when
we use the trivial / dummy implementation of resource group. The fix for
that is trivial (this commit). But it begs the question: should we make
the build system less magical (switching the implementation based on the
platform), and instead just always exercise the dummy implementation (or
at least the building of it).

src/backend/utils/resgroup/resgroup-ops-dummy.c

@@ -14,6 +14,7 @@
 
 #include "postgres.h"
 
+#include "utils/resgroup.h"
 #include "utils/resgroup-ops.h"
 
 /*