Fix build with -fno-common

Index: play-mod.c
--- play-mod.c.orig
+++ play-mod.c
@@ -50,7 +50,7 @@ roundToNote(i15x p)
 typedef u8 ModSample[30];
 typedef u8 ModNote[4];
 
-struct {
+static struct {
     const u8 *p0;
     const u8 *songName;
     i15x ordNum;
