Fix headerscheck in vpath builds

Oversight in dac048f71e per buildfarm animal crake.
Fix per suggestion from Andrew Dunstan.

Discussion: https://www.postgresql.org/message-id/e3f4a3d0-dfcc-41cc-1ed2-acc15700ddef%40dunslane.net

src/tools/pginclude/headerscheck

@@ -155,6 +155,8 @@ do
 		EXTRAINCLUDES="$python_includespec" ;;
 	    src/interfaces/ecpg/*)
 		EXTRAINCLUDES="-I $builddir/src/interfaces/ecpg/include -I $srcdir/src/interfaces/ecpg/include" ;;
+		src/backend/utils/adt/*)
+		EXTRAINCLUDES="-I $builddir/src/backend/utils/adt/" ;;
 	    *)
 		EXTRAINCLUDES="" ;;
 	esac