[soundmgr_openal] Pause/Resume Sound.

The following changes fixes a case for me where I hear the sound change levels up and down for each pause un-pause cycle.
Patch provided by daniel.c.wickstrom@gmail.com.

simgear/sound/soundmgr_openal.cxx

@@ -340,7 +340,7 @@ void SGSoundMgr::suspend()
         for ( auto current = d->_sample_groups.begin();
                    current != d->_sample_groups.end(); ++current ) {
             SGSampleGroup *sgrp = current->second;
-            sgrp->stop();
+            sgrp->suspend();
         }
         _active = false;
     }