res_fax_spandsp: Use g711_free() when available.

Per Johann Steinwendtner on the asterisk-dev mailing list:

http://lists.digium.com/pipermail/asterisk-dev/2014-March/066102.html

g711_free() was introduced in spandsp 0.0.6pre4 and g711_release() became a
noop.  I opted not to remove the call to g711_release() since it is harmless
and to call g711_free() if we have a sufficiently recent version of spandsp.

(issue ASTERISK-20149)
Reported by: Alexandr Gordeev


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@410829 f38db490-d61c-443f-a65b-d21fe96a405b