libpq: Add missing directory to installdirs target

It prevented the libpq directory from being installable by itself.

src/interfaces/libpq/Makefile

@@ -125,7 +125,7 @@ installcheck:
 	$(MAKE) -C test $@
 
 installdirs: installdirs-lib
-	$(MKDIR_P) '$(DESTDIR)$(includedir)' '$(DESTDIR)$(includedir_internal)'
+	$(MKDIR_P) '$(DESTDIR)$(includedir)' '$(DESTDIR)$(includedir_internal)' '$(DESTDIR)$(datadir)'
 
 uninstall: uninstall-lib
 	rm -f '$(DESTDIR)$(includedir)/libpq-fe.h'