remove useless extern declaration

[Jan Stary]

extern char *optarg is already declared in unistd.h
This is the only occurence in src/sbin and src/bin;
others will follow in separate mails.

        Jan


Index: bioctl.c
===================================================================
RCS file: /cvs/src/sbin/bioctl/bioctl.c,v
retrieving revision 1.139
diff -u -p -r1.139 bioctl.c
--- bioctl.c    21 Sep 2016 17:50:05 -0000      1.139
+++ bioctl.c    19 Oct 2016 21:43:43 -0000
@@ -59,7 +59,7 @@ struct timing {
        int             start;
 };
 
-void                   usage(void);
+static void __dead     usage(void);
 const char             *str2locator(const char *, struct locator *);
 const char             *str2patrol(const char *, struct timing *);
 void                   bio_status(struct bio_status *);
@@ -100,7 +100,6 @@ int
 main(int argc, char *argv[])
 {
        struct bio_locate       bl;
-       extern char             *optarg;
        u_int64_t               func = 0;
        char                    *devicename = NULL;
        char                    *realname = NULL, *al_arg = NULL;
@@ -273,7 +272,7 @@ main(int argc, char *argv[])
        return (0);
 }
 
-void
+static void __dead
 usage(void)
 {
        extern char             *__progname;