patch 8.1.1515: memory leak reported for sound when build with EXITFREE

Problem:    Memory leak reported for sound when build with EXITFREE.
Solution:   Free sound stuff when exiting.
diff --git a/src/misc2.c b/src/misc2.c
index 2ac7f5e..d1615b1 100644
--- a/src/misc2.c
+++ b/src/misc2.c
@@ -1247,6 +1247,9 @@
     /* screenlines (can't display anything now!) */
     free_screenlines();
 
+# if defined(FEAT_SOUND)
+    sound_free();
+# endif
 # if defined(USE_XSMP)
     xsmp_close();
 # endif