Make setenforce usage function static.

Change-Id: I6de204dc072418805160a35d799b8948ecb39251
diff --git a/toolbox/setenforce.c b/toolbox/setenforce.c
index 1b0ea5c..444073d 100644
--- a/toolbox/setenforce.c
+++ b/toolbox/setenforce.c
@@ -7,7 +7,7 @@
 #include <errno.h>
 #include <selinux/selinux.h>
 
-void usage(const char *progname)
+static void usage(const char *progname)
 {
     fprintf(stderr, "usage:  %s [ Enforcing | Permissive | 1 | 0 ]\n",
             progname);