arch/alpha/boot/tools/objstrip.c: Mark "usage" static

Nothing outside of arch/alpha/boot/tools/objstrip.c references the usage
function.
Signed-off-by: Josh Triplett <josh@joshtriplett.org>

arch/alpha/boot/tools/objstrip.c

@@ -35,7 +35,7 @@
 const char * prog_name;
 
 
-void
+static void
 usage (void)
 {
     fprintf(stderr,