Fix header inclusion issue.

partprune.h failed to compile by itself; needs to include partdefs.h.

I think I must've broken this in fa2cf164a, though I'd swear I ran
the appropriate tests when removing #includes.  Anyway, it's very
sensible for this file to include partdefs.h, so let's just do that.

Per cpluspluscheck.

src/include/partitioning/partprune.h

@@ -15,6 +15,8 @@
 #define PARTPRUNE_H
 
 #include "nodes/execnodes.h"
+#include "partitioning/partdefs.h"
+
 struct PlannerInfo;				/* avoid including pathnodes.h here */
 struct RelOptInfo;