Fixed bug in ant script to create dist directory if it is not there

$ ant dist.macosx
...
dist.macosx:
      [zip] Building zip: /Users/manuel/git/jitsi-videobridge/dist/macosx/jitsi-videobridge-macosx-build.SVN.zip

BUILD FAILED
/Users/manuel/git/jitsi-videobridge/build.xml:278: Problem creating zip: /Users/manuel/git/jitsi-videobridge/dist/macosx/jitsi-videobridge-macosx-build.SVN.zip (No such file or directory) (and the archive is probably corrupt but I could not delete it)