sound: fix a couple of implicit declarations

Include math.h to prevent implicit/incompatible declarations on round(3).

https://bugzilla.gnome.org/show_bug.cgi?id=669025

panels/sound/gvc-mixer-dialog.c

@@ -23,6 +23,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <unistd.h>
+#include <math.h>
 
 #include <glib.h>
 #include <glib/gi18n-lib.h>

panels/sound/gvc-stream-status-icon.c

@@ -23,6 +23,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <unistd.h>
+#include <math.h>
 
 #include <glib.h>
 #include <glib/gi18n.h>