{dir,song}vec: these structs are const

We definitely don't modify them here.

src/dirvec.c

@@ -6,7 +6,7 @@
 #include <string.h>
 #include <glib.h>
 
-static size_t dv_size(struct dirvec *dv)
+static size_t dv_size(const struct dirvec *dv)
 {
 	return dv->nr * sizeof(struct directory *);
 }

src/songvec.c

@@ -15,7 +15,7 @@ static int songvec_cmp(const void *s1, const void *s2)
 	return strcmp(a->url, b->url);
 }
 
-static size_t sv_size(struct songvec *sv)
+static size_t sv_size(const struct songvec *sv)
 {
 	return sv->nr * sizeof(struct song *);
 }