aix: fix misreading of condition in 8f12a4e

This lead to choosing the aix4.1 specific way of building the export file for
the backend, rather than the modern one.

Per buildfarm member hoverfly.

Discussion: https://postgr.es/m/20220807182707.gi7pirwbz5etprfo@awork3.anarazel.de

Author: anarazel

src/backend/Makefile

@@ -101,7 +101,7 @@ postgres: $(POSTGRES_IMP)
 
 $(POSTGRES_IMP): $(OBJS)
 	$(LD) $(LDREL) $(LDOUT) SUBSYS.o $(call expand_subsys,$^)
-	$(MKLDEXPORT) SUBSYS.o $(bindir)/postgres > $@
+	$(MKLDEXPORT) SUBSYS.o . > $@
 	@rm -f SUBSYS.o
 
 endif # aix