zfs_main.c: fix unused variable error with GCC

zfs_setproctitle_init() is stubbed out on FreeBSD.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Rob Wing <rob.fx907@gmail.com>
Closes #14441

cmd/zfs/zfs_main.c

@@ -8638,7 +8638,6 @@ main(int argc, char **argv)
 	int i = 0;
 	char *cmdname;
 	char **newargv;
-	extern char **environ;
 
 	(void) setlocale(LC_ALL, "");
 	(void) setlocale(LC_NUMERIC, "C");

include/libzutil.h

@@ -162,6 +162,7 @@ void color_end(void);
 int printf_color(const char *color, char *format, ...);
 
 #ifdef __linux__
+extern char **environ;
 _LIBZUTIL_H void zfs_setproctitle_init(int argc, char *argv[], char *envp[]);
 _LIBZUTIL_H void zfs_setproctitle(const char *fmt, ...);
 #else