patch 8.1.1851: crash when sound_playfile() callback plays sound

Problem:    Crash when sound_playfile() callback plays sound.
Solution:   Invoke callback later from event loop.
diff --git a/src/feature.h b/src/feature.h
index 89d6cae..64689c9 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -666,6 +666,9 @@
 #if !defined(FEAT_SOUND) && defined(HAVE_CANBERRA)
 # define FEAT_SOUND
 #endif
+#if defined(FEAT_SOUND) && defined(HAVE_CANBERRA)
+# define FEAT_SOUND_CANBERRA
+#endif
 
 /* There are two ways to use XPM. */
 #if (defined(HAVE_XM_XPMP_H) && defined(FEAT_GUI_MOTIF)) \