song: don't make the struct packed

The "packed" attribute may have negative side effects on performance.
Remove the "packed" attribute, and increase the size of "song.url" to
a multiple of the machine word size.

src/song.c

@@ -32,7 +32,7 @@ Song *
 song_alloc(const char *url, enum song_type type, struct _Directory *parent)
 {
 	size_t urllen = strlen(url);
-	Song *song = xmalloc(sizeof(Song) + urllen);
+	Song *song = xmalloc(sizeof(*song) - sizeof(song->url) + urllen + 1);
 
 	song->tag = NULL;
 	memcpy(song->url, url, urllen + 1);

src/song.h

@@ -40,7 +40,7 @@ typedef struct _Song {
 	struct tag *tag;
 	struct _Directory *parentDir;
 	time_t mtime;
-	char url[1];
+	char url[sizeof(size_t)];
 } mpd_packed Song;
 
 Song *