Summary: comms/svxlink: Fix build with libc++ 3.8.0

During the exp-run in bug 208158, it was found that comms/svxlink gives errors
with libc++ 3.8.0 [1]:

/wrkdirs/usr/ports/comms/svxlink/work/svxlink-15.11/src/async/audio/AsyncAudioDe
+viceAlsa.cpp:551:7:
error: call to 'abs' is ambiguous
  if (::abs(real_rate - sample_rate) > 100)
      ^~~~~

This is because abs() is being called with an unsigned argument.  Fix this by
casting 'real_rate' to int, since it was assigned from 'sample_rate' anyhow.

PR:		ports/209371
Reported by:	@dim

comms/svxlink/files/patch-src_async_audio_AsyncAudioDeviceAlsa.cpp

@@ -0,0 +1,11 @@
+--- src/async/audio/AsyncAudioDeviceAlsa.cpp.orig	2015-11-22 16:03:59 UTC
++++ src/async/audio/AsyncAudioDeviceAlsa.cpp
+@@ -548,7 +548,7 @@ bool AudioDeviceAlsa::initParams(snd_pcm
+     return false;
+   }
+ 
+-  if (::abs(real_rate - sample_rate) > 100)
++  if (::abs((int)real_rate - sample_rate) > 100)
+   {
+     cerr << "*** ERROR: The sample rate could not be set to "
+          << sample_rate << "Hz for ALSA device \"" << dev_name << "\". "