Fixed Mac build breaker: failing ln -s command.

viewer_manifest.py has acquired new 'ln -s' commands to avoid duplicating
shared .dylib files between the main Second Life app bundle and the embedded
mac-updater and mac-crash-logger app bundles. Unfortunately, the second time I
ran with these new commands, they failed because the target already existed.
Added -f switch to make that case benign.

indra/newview/viewer_manifest.py

@@ -582,11 +582,11 @@ def construct(self):
                                     "libaprutil-1.0.3.8.dylib",
                                     "libexpat.0.5.0.dylib"):
                         target_lib = os.path.join('../../..', libfile)
-                        self.run_command("ln -s %(target)s '%(link)s'" % 
+                        self.run_command("ln -sf %(target)s '%(link)s'" % 
                                          {'target': target_lib,
                                           'link' : os.path.join(mac_crash_logger_res_path, libfile)}
                                          )
-                        self.run_command("ln -s %(target)s '%(link)s'" % 
+                        self.run_command("ln -sf %(target)s '%(link)s'" % 
                                          {'target': target_lib,
                                           'link' : os.path.join(mac_updater_res_path, libfile)}
                                          )