darwin-user: Add missing static attribute

Function usage is only used locally, so add "static".
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

darwin-user/main.c

@@ -704,7 +704,7 @@ void cpu_loop(CPUX86State *env)
 }
 #endif
 
-void usage(void)
+static void usage(void)
 {
     printf("qemu-" TARGET_ARCH " version " QEMU_VERSION ", Copyright (c) 2003-2004 Fabrice Bellard\n"
            "usage: qemu-" TARGET_ARCH " [-h] [-d opts] [-L path] [-s size] program [arguments...]\n"