[gnome-desktop-testing] Makefile: add -f to ginsttest-runner symlink creation

[Ray Strode <halfline src gnome org>]

commit 355287f65584f817f699dae09b4a934b0f711863
Author: Ray Strode <rstrode redhat com>
Date:   Thu Dec 6 09:56:51 2018 -0500

    Makefile: add -f to ginsttest-runner symlink creation
    
    Without this, you can't run make install more than once.

 Makefile.am | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/Makefile.am b/Makefile.am
index 5f0a16c..8a3571c 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -40,7 +40,7 @@ include Makefile-tests.am
 install-data-hook: $(INSTALL_DATA_HOOKS)
 
 install-exec-hook:
-       ln -s gnome-desktop-testing-runner $(DESTDIR)$(bindir)/ginsttest-runner
+       ln -sf gnome-desktop-testing-runner $(DESTDIR)$(bindir)/ginsttest-runner
 
 release-tag:
        git tag -m "Release $(VERSION)" v$(VERSION)