slirp: replace a fprintf with g_critical()

Reduce dependency on QEMU. QEMU could use a custom glib log handler if
it wants to redirect/filter it.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

slirp/socket.c

@@ -285,7 +285,7 @@ err:
 
     sofcantrcvmore(so);
     tcp_sockclosed(sototcpcb(so));
-    fprintf(stderr, "soreadbuf buffer to small");
+    g_critical("soreadbuf buffer too small");
     return -1;
 }