tools: use program_invocation_short_name provided by libc

Thanks to Dave Reisner for pointing this out.
diff --git a/tools/insmod.c b/tools/insmod.c
index f358b93..427495a 100644
--- a/tools/insmod.c
+++ b/tools/insmod.c
@@ -40,7 +40,7 @@
 		"Options:\n"
 		"\t-V, --version     show version\n"
 		"\t-h, --help        show this help\n",
-		binname);
+		program_invocation_short_name);
 }
 
 static const char *mod_strerror(int err)