osmo_io: Don't make msg in write_cb const

This was discussed in a previous change, but the change was merged
as-is.

Change-Id: I8b4a2dd7a336dea5c52c9da6e78bbc4d5f2a02f8
diff --git a/tests/osmo_io/osmo_io_test.c b/tests/osmo_io/osmo_io_test.c
index 1991926..bb99b54 100644
--- a/tests/osmo_io/osmo_io_test.c
+++ b/tests/osmo_io/osmo_io_test.c
@@ -49,7 +49,7 @@
 	talloc_free(msg);
 }
 
-static void write_cb(struct osmo_io_fd *iofd, int rc, const struct msgb *msg)
+static void write_cb(struct osmo_io_fd *iofd, int rc, struct msgb *msg)
 {
 	printf("%s: write() returned rc=%d\n", osmo_iofd_get_name(iofd), rc);
 }
@@ -109,7 +109,7 @@
 	talloc_free(msg);
 }
 
-static void sendto_cb(struct osmo_io_fd *iofd, int rc, const struct msgb *msg,
+static void sendto_cb(struct osmo_io_fd *iofd, int rc, struct msgb *msg,
 		      const struct osmo_sockaddr *daddr)
 {
 	printf("%s: sendto() returned rc=%d\n", osmo_iofd_get_name(iofd), rc);