Fix ant dependency on Debian

the ant package must be explicitly specified, otherwise the following error will raise when building osv. 

raphaelsc@debian:~/Desktop/osv_dir/osv$ make
  ANT tests/bench
Traceback (most recent call last):
  File "scripts/silentant.py", line 14, in <module>
    stderr = subprocess.PIPE)
  File "/usr/lib/python2.6/subprocess.py", line 623, in __init__
    errread, errwrite)
  File "/usr/lib/python2.6/subprocess.py", line 1141, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory
make: *** [all] Error 1

Signed-off by: Raphael S. Carvalho <raphael.scarv@gmail.com>